The Ancestor's Tale
is cast in the form of an epic pilgrimage from the present to the past.
All roads lead to the origin of life. But because we are human, the path
we shall follow will be a human pilgrimage to discover human ancestors.
As we go, we shall greet other pilgrims who join us at a series of rendezvous
points, as we encounter the common ancestor we share with each of them.
The first fellow pilgrims we greet, some five million years ago, deep in Africa where Stanley memorably shook hands with Livingstone, are the chimpanzees. A million years further into the past, the gorillas join us, then the orang utans. Next the gibbons, then monkeys ... and so on until we finally greet the bacteria, after which all the pilgrims march together in one single backward quest for the origin of life itself, life's "Canterbury". Following Chaucer's lead, my pilgrims, which are all the different species of living creature, have the opportunity to tell tales along the way. It is these tales that form the main substance of the book.
~ Introduction - The
Conceit of Hindsight
~ The Pilgrimage Begins - All Humankind
~ Rendezvous #1...#9
~ Rendezvous #10...#15
~ Rendezvous #16...#20
~ Rendezvous #20...#25
~ Rendezvous #26 - The Grasshopper's Tale
~ Rendezvous #27...#39
~ The Host's Return
INTRODUCTION - THE CONCEIT OF HINDSIGHT
"History doesn't repeat
itself, but it rhymes."
- Mark Twain
The historian is tempted to scour the past for patterns that repeat themselves; or, at least, to seek reason and rhyme for everything... the second connected temptation is the vanity of the present: of seeing the past as aimed at our own time, as though the characeter's in history's play had nothing better to do with their lives than foreshadow us.
Evolution rhymes, patterns recur. And this just doesn't happen to be so. It is so for well understood reasons: Darwinian reasons mostly, for biology, unlike human history or even physics, already has its grand unifying theory, accepted by all informed practitioners, though in varying versions and interpretations.
Were we to meet a Homo erectus face to face, it might well look to our eyes like an unfinished sculpture in the making. But that is only because we are looking with human hindsight. A living creature is always in the business of surviving in its own environment. It is never unfinished - or, in another sense, it is always unfinished. So, presumably, are we.
It makes no more sense (and no less) to aim our historical narrative towards Homo sapiens than towards any other modern species. It is a conceit of hindsight to see evolution as aimed towards some particular end point, such as ourselves. A historically minded swift, understandably proud of flight as self-evidently the premier accomplishment of life, might regard swiftkind - those spectacular flying machines with their swept-back wings, who stay aloft for a year at a time and even copulate in free flight - as the acme of evolutionary progress. If elephants could write history they might portray tapirs, elephant shrews, elephant seals and proboscis monkeys as tentative beginners along the main trunk road of evolution, taking the first fumbling steps but each - for some reason - never quite making it: so near yet so far. Elephant astronomers might wonder whether, on some other world, there exist alien life forms that have crossed the nasal rubicon and taken the final leap to full proboscitude.
We are neither swifts
nor elephants, we are people. As we wander in imagination through some
long dead geologicial epoch, it is humanly natural to reserve a special
warmth and curiosity for whichever otherwise ordinary species in that ancient
landscape is our ancestor (it is an intriguingly unfamiliar thought that
there is always one such species). It is hard to deny our human temptation
to see this one species as "on the main line" of evolution, the others
as supporting cast, walk-on parts, sidelined cameos. Without succumbing
to that error, there is one way to indulge a legitimate human-centrism
while respecting historical propriety. That way is to do our history backwards.
Backward chronology in search of ancestors really can sensibly aim towards
a single distant target. The distant target is the grand ancestor of all
life, and we can't help converging upon it no matter where we start - elephant
or eagle, swift or salmonella, wellingtonia or woman.
Backward chronology and forward chronology are each good for different purposes. Go backwards and no matter where you start, you end up celebrating the unity of life. Go forwards and you extol diversity.
In a backward chronology, the ancestors of any set of species must eventually meet at a particular geological moment. Their point of rendezvous is the last common ancestor that they all share, what I shall call their 'Concestor': the focal rodent or the focal mammal or the focal vertebrate, say. The oldest concestor is the grand ancestor of all surviving life.
The grand confluence of all surviving life is not the same thing as the origin of life itself. This is because all surviving species presumably share a concestor who lived after the origin of life: anything else would be an unlikely coincidence, for it would suggest that the original life form immediately branched and more than one of its branches survive to this day.
If every fossil were magicked away, the comparitive study of modern organisms, of how their patterns of resemblances, especially of their genetic sequences, are distributed among species, and of how species are distributed among continents and islands, would still demonstrate, beyond all sane doubt, that our history os evolutionary, and that all living creatures are cousins. Fossils are a bonus. It is worth remembering this when creationists go on about 'gaps' in the fossil record. The fossil record could be one big gap, and the evidence for evolution would still be overwhelmingly strong.
THE PILGRIMAGE BEGINS
Our situation as hunter-gatherers
cannot have been a Utopia either. It has lately become fashionable to regard
hunter-gatherers and primitive agricultural societies as more 'in balance'
with nature than us. This is probably a mistake. They may well have had
greater knowledge of the wild, simply because they lived and survived in
it. But, like us, they seem to have used their knowledge to exploit (and
often overexploit) the environment to the best of their abilities at the
time. Far from being in balance with nature, pre-agricultural hunter-gatherers
were probably responsible for widespread extinctions of many large animals
around the globe. Just prior to the Agricultural Revolution, the colonisation
of remote areas by hunter-gatherer peoples is suspiciously often followed
in the archaeological record by the wiping out of many large (and presumably
palatable) birds and mammals.
We tend to regard 'urban' as the antithesis of 'agricultural' but in the longer perspective that this book must adopt, city dwellers should be lumped in with farmers as opposed to hunter-gatherers.
- from The Farmer's Tale
Throughout this book
I use 'primitive' in the technical sense, to mean 'more like the ancestral
state'. No implication of inferiority is intended.
- note from The Farmer's Tale
It is a remarkable
fact that there must be a moment in history when there were two animals
in the same species, one of whom became the ancestor of all humans and
no aardvarks, while the other became the ancestor of all aardvarks, and
no humans. They may well have met, and may even have been brothers. You
can cross out aardvark and substitute any other modern species you like,
and the statement must still be true. Bear in mind when you do so that
the 'ancestor of all aardvarks' will also be the ancestor of lots of very
different things besides aardvarks.
- from Rendezvous 0: All Humankind
Individual people who
are closely related share a large number of gene trees. We share athe majority
of our gene trees with our close kind. But some gene trees deliver a 'minority'
vote, placing us closer to out otherwise more distant relatives. We can
think of closeness of kinship among people as a kind of majority vote among
genes. Some of your genes vote for, say, the Queen, as a close cousin.
Others argue that you are closer to seemingly much more distant individuals
(as well shall see, even members of other species). When quizzed, each
piece of DNA has a different view of what history is all about, because
each has blazed a different path through the generations.
- from Eve's Tale
"An itinerant selfish
gene said 'Bodies a-plenty I've seen.
You think you're so clever but I'll live for ever. You're just a survival machine."
- the message of 'The Selfish Gene' from Eve's Tale
If only one Neanderthal
male, say, bred into a sapiens population, that gave him a reasonable chance
of being a common ancestor to all Europeans alive today. This can be true
even if Europeans contain no Neanderthal genes at all. A striking thought.
So although few, if any, of our genes come from Neanderthals, it is possible
that some people have many Neanderthal ancestors... this is the distinction
between gene trees and people trees. Evolution is governed by the flow
of genes. The words 'evolutionary descent' refer to gene ancestors, not
- from The Neanderthal's Tale
Big animals need to
be a different shape from small animals, if only because of the area/volume
scaling rules we have just seen. If you turned a shrew into an elephant
just by inflating it, retaining the same shape, it wouldn't survive. A
shrew scaled up to elephant size would have spindly legs that would break
under the strain, and its slender muscles would be too weak to work. The
strength of a muscle is proportional not to its volume but to its cross-sectional
area... we go into this through thinking about brain size. We can't just
compare our brains with those of Homo habilis or any other species without
making allowance for body size.
- from The Handyman's Tale
The popular literature
on human fossils is hyped up with alleged ambition to discover the 'earliest'
human ancestor. This is silly. You can ask a specific question like 'Which
was the earliest human ancestor to walk habitually on two legs?' or 'Which
was the first creature to be our ancestor and not the ancestor of a chimpanzee?'
or 'Which was the earliest human ancestor to have a brain volume larger
than 600cc?' Those questions at least mean something in principle, although
they are hard to answer in practice and some of them suffer from the vice
of erecting artificial gaps in a seamless continuum. But 'Who was the earliest
human ancestor?' means nothing at all.
- from Ape Men
The idea (I attribute
this inspired way of expressing it to Steven
Pinker) is that before the invention of the freezer the best larder
for meat was a companion's belly. How so? The meat itself was no longer
available, of course, but the goodwill it buys is safe in long-term storage
in a companion's brain. Your companion will remember the favour and repay
it when fortunes are reversed. Chimpanzees are known to share meat for
favours. In historic times, this kind of IOU became tokenized as money.
- from Little Foot's Tale
RENDEZVOUS #1 - #9
In our fantasy
the chimpanzee pilgrims meet us in some Pliocene forest clearing, and their
dark brown eyes, like our less predictable ones, are fixed upon Concestor
1: their ancestor as well as ours... we muct not assume, as many laymen
do, that our ancestors were chimpanzees. Indeed, the very phrase 'missing
link' is suggestive of this misunderstanding. You still hear people saying
things like, 'Well, if we are descended from chimpanzees, why are there
still chimpanzees around?' So, when we and the chimpanzee\bonobo pilgrims
meet at the rendezvous point, the likelihood is that the shared ancestors
that we greet was hairy like a chimpanzee, and had a chimpanzee-sized brain.
- from Rendezvous 1: Chimpanzees
Bonobos use sex as
a currency of social interaction, somewhat as we use money. They use copulation,
or copulatory gestures, to appease, to asset dominance, to cement bonds
with other troops members of any age or sex, including smal infants. Paedophilia
is not a hang-up with bonobos: all kinds of philia seem fine to them.
- from The Bonobo's Tale
Species are composites
of DNA from many different sources. A Marxian taxonomist shown only the
genitals of a male human, a female human, and a male gibbon would have
no hesitation in classifying the two males as more closely related to each
other than either is to the female. Indeed, the genes determining maleness
(called SRY) has never been in a female body, at least since long before
we and the gibbons diverged.
- from The Gibbon's Tale
Usually, in everyday
life, massive improbability is a good reason for thinking that something
won't happen. The point about intercontinental rafting of monkeys, or rodents
or anything else, is that it only had to happen once, and the time available
for it to happen, in order to have momentous consequences, is way outside
what we can grasp intuitively. The odds against a floating mangrove bearing
a pregnant female monkey and reaching a landfall in any one year may be
ten thousand to one against. That sounds tantamount to impossible by the
lights of human experience. But given 10 million years it becomes almost
- from Rendezvous 6: New World Monkeys
There are anecdotes
of bomber crews in the Second World War deliberately recruiting one colourblind
member because he could spot certain types of camouflage better than his
otherwise more fortunate trichromat comrades. Experimental evidence confirms
that human dichromats can indeed break certain forms of camouflage that
fool trichromats. Is it possible that a troop of monkeys consisting of
trichromat and two kinds of dichromats might collectively find a greater
variety of fruits than a troop of pure trichromats? This might sound far-fetched,
but it is not silly.
- from The Howler Monkey's Tale
"The aye-aye is a nocturnal
lemur. It is a very strange-looking creature that seems to have been assembled
from bits of other animals. It looks a little like a large cat with a bat's
ears, a beaver's teeth, a tail like a large ostrich feather, a middle finger
like a long dead twig and enormous eyes that seem to peer past you into
a totally different world which exists just over your left shoulder...
Like virtually everything that lives on Madagascar, it does not exist anywhere
else on earth."
- quoting from "Last Chance to See" by Douglas Adams in The Aye-Aye's Tale
For a biologist, Madagascar
is the Island of the Blest. Along our pilgrim voyage, it is the first of
five large - in some cases very large indeed - islands, whose isolation,
at crucial junctures in Earth history, radically structured the diversity
of mammals. The islands or island continents that have shaped the evolution
of mammals are, in the order we shall visit them, Madagascar, Laurasia,
South America, Africa and Australia.
- from The Aye-Aye's Tale
A taxonomist might
argue that, for reasons of convenience in exhibiting museum specimens,
tree shrews should be grouped with shrews and colugos with flying squirrels.
In such judgements there is absolutely no right answer. The phyletic taxonomy
adopted in this book is different. There is a correct tree of life, but
we don't yet know what it is. There is still room for human judgement,
but it is judgement about that will eventually turn out to be the undisputable
truth. It is only because we haven't looked at enough details yet, especially
molecular details, that we are still unsure about what that truth is. The
truth really is hanging up there waiting to be discovered. The same cannot
be said for judgements of taste or of museum convenience.
- from The Colugo's Tale
RENDEZVOUS #10 - #15
Rodents are one of
the great success stories of mammaldom. More than 40% of all mammal species
are rodents, and there are said to be more individual rodents in the world
than all other mammals combined. Rats and mice have been the hidden beneficiaries
of our own Agricultural Revolution, and tthey have travelled with us across
the seas to every land in the world... When even the four horseman are
laid low by the apocalypse, rats will swarm like lemmings over the ruins
Most rodents are mouse-sized, but they range up through marmots, beavers, agoutis and maras to the sheep-sized capybaras of the South American waterways. Large as they are, modern capybaras are dwarfed by various giant South American rodents what went extinct only quite recently. Telicomys was an even larger rodent the size of a small rhinoceros which went extinct at the time of the Great American Interchange, when the Isthmus of Panama ended South America's island status.
A world without rodents would be a very different world. It is less likely to come to pass than a world dominated by rodents and free of people. If nuclear war destroys humanity and most of the rest of life, a good bet for survival in the short term, and for evolutionary ancestry in the long term is rats. I have a post-Armageddon vision. We and all other large animals are gone. Rodents emerge as the ultimate post-human scavengers. They gnaw their way through New York, London and Tokyo... within 5 million years, a whole range of new species replace the ones we know. Herds of giant grazing rats are stalked by sabre-toothed predatory rats. Given enough time, will a species of intelligent, cultivated rats emerge?
- from Rendezvous 10: Rodents and Rabbitkind
My first book 'The
Selfish Gene', could equally have been called 'The Co-Operative Gene' without
a word of the book itself needed to be changed. Indeed, this might have
saved some misunderstanding (some of a book's most vocal critics are content
to read the book by title only). Selfishness and co-operation are two sides
of a Darwinian coin. Each gene promotes its own selfish welfare, by co-operating
with other genes in the sexually stirred gene pool which is that gene's
environment, to build shared bodies.
- from The Beaver's Tale
I suspect that major
new departures in evolution often start in just such a way, with a piece
of lateral thinking by an individual who discovers a new and useful trick,
and learns to perfect it. If the habit is then imitated by others, including
perhaps the individual's own children, there will be a new selection pressue
set up. Natural selection will favour genetic predispositions to be good
at learning the new trick, and much will follow. I suspect that something
like this is how 'instinctive' feeding habits such as tree-hammering in
woodpeckers, and mollusc-smashing in thrushes and sea otters, got their
- from The Hippo's Tale
in the history of the whales that made them flip into evolutionary overdrive.
They evolved so much faster than all the rest of the artiodactyls that
their origin within that group was obscured, until molecular taxonomists
came along and uncovered it. So, what is special about the history of the
whales? Leaving the land and becoming wholly aquatic was a bit like going
into outer space. Think of a whale as what a hippo would be like if only
it could be freed from the tyranny of gravity.
- from The Hippo's Tale
The separate optimisation
of male and female characteristics comes about through selection of genes.
People are sometimes surprised to learn that the genes concerned are present
in both sexes. Natural selection has favoured so-called sex-limited genes.
Sex-limited genes are present in both sexes but turned on in only one sex.
For example, genes that tell the developing seal: 'If you are male grow
very big and fight' are favoured at the same time as genes that say: 'If
you are female grow small and don't fight'. Both classes of genes are passed
on to sons and to daughters, but each is expressed in one sex and not the
- from The Seal's Tale
South America is a sort of giant Madagascar. Like Madagascar, it split
off from Africa, but from the west rather than the east side, around the
same time, or a bit later, than Madagascar. Like Madagascar, South America
was cut off from the rest of the world during most of the period of mammal
evolution. Its long purdah, which ended only about 3 million years ago,
led to South America becoming a gigantic natural experiment culminating
in a unique and fascinating mammal fauna. Like Australia but unlike Madagascar,
South America's fauna was rich in marsupials. In South America's casem
marsupials filled most of the carnivorous niches.
Unlike Madagascar and Australia, South America's isolation came to an end naturally, before human travel brought all zoological isolation more or less to an end. The separate faunas of North and South America were free to travel along the narrow corridor formed by the rise of the Isthmus of Panama to enter each other's continents. This enriched the two faunas, but them some extinctions occured on both sides, presumably at least partly as a result of competition.
- from The Armadillo's Tale
In spite of their dear
little trunks, it never occured to anyone that elephant shrews might be
particularly close to elephants. It was always assumed that they were just
African versions of Eurasian shrews. Recent molecular evidence, however,
astonishes us with the information that elephant shrews are closer cousins
to elephants than they are to shrews, and some people now prefer their
alternative nam, sengi, to distance them from shrews.
- from Rendezvous 13: Afrotheres
Animals often behave
as if they know what is good for them in the future, but we must be careful
not to forget that 'as if'. Natural selection if a beguiling counterfeiter
of deliberate purpose.
- note from Rendezvous 13: Afrotheres
Thylacinus, the Tasmanian
wolf, is one of the most famous examples of convergent evolution. Thylacines
are sometimes called Tasmanian tigers because of their striped backs, but
it is an unfortunate name. They are much more like wolves or dogs. They
are easy to tell from a true dog because of the stripes on their back but
the skeleton is harder to distinguish. Zoology students of my generation
at Oxford had to identify 100 zoological specimens as part of the final
exam. Word soon got around that, if ever a 'dog' skull was given, it was
safe to identify it as Thylacinus on the ground that anything as obvious
as a dog skull had to be a catch. Then one year the examiners, to their
credit, double bluffed and put in a real dog skull.
Dingos, of course, are not marsupials but real dogs, probably introduced by aboriginal man. It may have been partly competition from dingos that drove thylacines extinct on mainland Australia.
- from The Marsupial Mole's Tale
The platypus bill seems
comic, its resemblance to that of a duck made the more incongruous by its
relatively large size, and also because a duck's bill has a certain intrinsic
laughableness. But humour does an injustice to this wondrous apparatus.
If you want to think in terms of an incongruous graft, forget all about
ducks. A more telling comparison is the extra nose grafted onto a Nimrod
reconaissance aircraft... Platypuses have about 40000 electrical sensors
distributed in longitudinal stripes over both surfaces of the bill... they
swing the bill in movements called saccades, side to side, as they swim.
It looks like a radar dish scanning... when you think of a platypus, forget
duck, think Nimrod, think AWACS; think huge hand feeling its way, by remote
pins and needles; think lightning flashes and thunder rumbling, through
the watery mud of Australia.
- from The Duckbill's Tale
The more important
moral of this tale is that even an animal what is genuinely primitive in
all respects is primitive for a reason. The ancestral characteristics are
good for its way of life, so there is no reason to change. As Professor
Arthur Cain of Liverpool University used to say, an animal is the way it
is because it needs to be.
- from The Duckbill's Tale
RENDEZVOUS #16 -#20
Birds are an offshoot
of one particular order of dinosaurs, the saurischians. The saurischian
dinosaurs, such as Tyrannosaurus and the gigantic sauropods, are closer
to birds than they are to the other main group of dinosaurs, the unfortunately-named
ornithischians, such as Iguanadon, Triceratops and the duckbilled hadrosaurs.
Tyrannosaurus are closer cousins to birds than they are even to other saurischians
such as the large plant-eating sauropods Diplodocus and Brachiasaurus.
These, then, are the sauropsid pilgrims, the turtles, lizards and snakes,
crocodiles, and birds, together with the huge concourse of shadow pilgrims
- the pterosaurs in the air, the ichthyosaurs, pleisosaurs and mosasaurs
in the water, and above all the dinosaurs on land.
- Rendezvous 16: Sauropsids
Why does the male twelve-wired
bird of paradise have red eyes and a black ruff with an iridescent green
fringe, while Wilson's bird of paradise catches the eye with a scarlet
back, yellow neck and blue head? Not because something in their respective
diets or habitats predisposes these two species to their different colour
schemes. No, these differences, and those that so conspicuously mark out
all other species of bird of paradise, are arbitrary, whimsical, unimportant
to anybody - except female birds of paradise. Sexual selection does this
kind of thing. Sexual selection produces quirky, whimsical evolution that
runs away in apparently arbitrary directions, feeding on itself to produce
wild flights of evolutionarty fancy. On the other hand, sexual selection
also tends to magnify differences between sexes: sexual dimorphism. Any
theory that attributes human brains, bipedality or nakedness to sexual
selection has to face up to a major difficulty. There is no evidence that
one sex is brainier than the other, nor that one sex is more bipedal than
- from The Peacock's Tale
In post-Wallace language,
a Wallacean female is, in effect, reading a male's genes by their external
manifestations from which she judges their quality. And it is a startling
consequence of some sophisticated neo-Wallacean theorising that males are
expected to go out of their way to male it easy for females to read their
quality, even if their quality if poor. This brings us to the first of
our three questions about human evolution. Why did we lose our hair? Mark
Pagel and Walter Bodmer have made the intriguing suggestion that hairlessness
evolved to reduce ectoparasites such as lice and, in keeping with the theme
of this talem as a sexually selected advertisement of freedom of parasites...
Nakedness is not only bad news for lice and ticks. It is good news for
choosers trying to discover whether a would-be sexual partner has lice
- from The Peacock's Tale
I know Michelangelo
might not, as it happens, have been interested in impressing females. It
is still entirely plausible that his brain was 'designed' by natural selection
for impressing females, just as - whatever his personal preferences - his
penis was designed for impregnating them. The human mind, on this view,
is a mental peacock's tail. And the brain expanded under the same kind
of sexual selection as drove the enlargement of the peacock's tail.
- from The Peacock's Tale
It is hard for a land
animal to reach an island, but it is a lot easier if it has wings. Flying
animals are in a special situation. Their wings carry them, perhaps as
a freak accident, blown on a gale. Having arrived on wings, they find that
they no longer need them. Especially because islands often lack predators.
This is why island animals, as Darwin noted on the Galapagos, are often
remarkably tame. And it makes them easy meat for sailors. The most famous
example is the dodo, Raphus cucullatus, cruelly named Didus ineptus by
Linnaeus, the father of taxonomy.
The ancestors of the dodos had wings. They took a long time to evolve, why not hang on to them in case one day they might come in useful again? Alas (for the dodo) that is not the way evolution thinks.
Evolution, or its driving engine natural selection, has no foresight. In every generation within every species, the individuals best equipped to survive and reproduce contribute more than their fair share of genes to the next generation. The consequence, blind as it is, is the nearest approach to foresight that nature admits. Wings might be useful a million years hence when sailors arrive with clubs. But wings will not help a bird contribute genes to the next generation, in the immediate here and now. On the contrary, wings, and especially the massive breast muscles needed to power them, are an expensive luxury. Shrink them, and the resources saved can now be spent on something more immediately useful such as eggs: immediately useful for surviving and reproducing the very genes that programmed the shrinkage.
That's the kind of thing natural selection does all the time. Survival in future centuries doesn't enter into the calculation, for the good reason that it isn't really a calculation at all. It all happens automatically, as some genes survive in the gene pool and others don't.
Ancestral flying birds are carried by their wings to a remote island where an absence of mammals opens up opportunities for making a living on the ground. Their wings are no longer useful in the way that they were on the mainland, so the birds give up flying, and their wings and costly wing muscles degenerate. Birds arrive by air, then natural selection grounds them to fill mammalian gaps in the market.
- from The Dodo's Tale
150 million years ago,
Gondwanaland consisted of everything that we now know as South America,
Africa, Arabia, Antartica, Austalasia, Madagascar and India. The southern
tip of Africa was touching Antartica, and tilted to the 'right'. There
was therefore a triangular gap between the east coast of Africa and the
north coast of Antartica - but it wasn't really a gap because it was filled
by India. India was in those days separated from the rest of Asia (Laurasia)
by an ocean, the Tethys. But Gondwanaland was about to break up...
The legend of the roc, the fabulous great bird with the strength to shift elephants, is a wonder of childhood. But isn't the true story of how the very continents themselves are shifted, through thousands of miles, and even greater wonder, more worthy of the adult imagination?
- from The Elephant Bird's Tale
If you follow the population
of herring gulls westward to North America, then on around the world to
Siberia and back to Europe again, you notice a curious fact. The 'herring
gulls', as you move round the pole, gradually become less and less like
herring gulls and more and more like lesser black-bakced gulls until it
turns out that our Western European lesser black-backed gulls actually
are at the other end of a ring-shaped continuum which started with herring
gulls. At every stage of the ring, the birds are sufficiently similar to
their immediate neighbours in the ring to interbreed with them. Until that
is, the ends of the continuum are reached, and the ring bites itself in
the tail. The herring gulls and the lesser black-backed gull in Europe
never interbreed, although they are linked by a continuous series of interbreeding
colleagues all the way round the other side of the world. Ring species
like the salamanders and the gulls are only showing us in the spatial dimension
something that must always happen in the time dimension.
People and chimpanzees are certainly linked via a continuous chain of intermediaries and a shared ancestor, but the intermediaries are extinct: what remains is a discontinuous distribution. The same if true of people and monkeys, and of people and kangaroos, except that the extinct intermediaries lived longer ago. Because the intermediaries are almost always extinct, we can usually get away with assuming there is a sharp discontinuity between every species and every other. But in this book we are concerned with evolutionary history, with the dead as well as the living.
Cats and dogs have evolved from a common ancestor who lived tens of millions of years ago. If only the intermediaries were still alive, attempting to separate cats from dogs would be a doomed enterprise, as it is with the salamanders and the gulls. In a world of perfect and complete information, fossil information as well as recent, discrete names for animals would become impossible. Instead of discrete names we would need sliding scales, just as the words hot, warm, cool and cold are better replaced by a sliding scale such as Celsius or Fahrenheit.
Everyone agrees that Homo sapiens is a different species from Pan troglodytes, the chimpanzee. But everyone also agres that if you follow human ancestry backward to the shared ancestor and then forward to chimpanzees, the intermediaries all along the way will form a gradual continuum in which every generation would have been capable of mating with its parent or child of the opposite sex.
- from The Salamander's Tale
To call Homo erectus
a separate species from Homo sapiens could have a precise meaning in principle,
even if it is impossible to test in practice. It means that if we could
go back in our time machine and meet our Homo erectus ancestors, we could
not interbreed with them. But suppose that, instead of zooming directly
to the time of Homo erectus, we stopped our time machine every thousand
years to pick up a young and fertile passenger... there would be no biological
barrier to her interbreeding with a member of the opposite sex from 1000
years earlier... the daisy chain would continue on back to when our ancestors
were swimming in the sea. Yet although we couldn't interbreed with them,
we are linked by an unbroken chain of ancestral generations, every one
of which could have interbred with their immediate predecessors and immediate
successors in the chain.
There would never be a generation in which it made sense to say of an individual that he is Homo sapiens but his parents are Homo erectus. There is no reason to think that any child was ever a member of a different species from its parents, even though the daisy chain of parents and children stretches back from humans to fish and beyond. It is no more paradoxical than the statement that there is never a moment when a growing child ceases to be short and becomes tall, or a kettle ceases to be cold and becomes hot.
Creationists love 'gaps' in the fossil record. Little do they know, biologists have good reasont yo love them too. Without gaps in the fossil record, our whole system for naming species would break down. Fossils could not be given names, they'd have to be given numbers or positions on a graph.
Perhaps because our brains evolved in world where most of the intermediaries between living species are dead, we often feel more comfortable if we can use separate names for things when we talk about them... let us use names as if they really reflected a discontinuous reality, but by all means let's privately remember that, at least in the world of evolution, it is no more than a convenient fiction, a pandering to our own limitations.
- from The Salamander's Tale
Paedomophosis is one
of those ideas of which, once you get the hang of it, you start seeing
examples everywhere you look. What does an ostrich remind you of? The wings
of an ostrich are silly little stubs, just like the wings of a newly hatched
chick. Instead of the stout quills of a flying bird, ostrich feathers are
coarse versions of the fluffy down of a baby chick. An ostrich is an overgrown
- from The Axolotl's Tale
Natural selection penalises
mating with the wrong species, especially where the species are close enough
for it to be a temptation, and close enough for hybrid offspring to survive,
to consume costly parental resources, and then turn out to be sterile,
like mules... it is probably correct to interpet some courtship displays
and other conspicuous advertisements, as 'reproductive isolation mechanisms'
evolved through selection against hybridisation.
- note from The Cichlid's Tale
RENDEZVOUS #26: THE GRASSHOPPER'S TALE
The Grasshopper's Tale treats of the vexed and sensitive topic of race...
"Race" is not a clearly defined word. "Species" is different. There really is an agreed way to decide whether two animals belong in the same species: can they interbreed? The interbreeding criterion gives the species a unique status in the hierarchy of taxonomic levels... The interbreeding criterion works pretty well, and it delivers an unequivocal verdict on humans and their supposed races. All living human races interbreed with one another. We are all members of the same species, and no reputable biologist would say any different. But let me call your attention to an interesting, perhaps even slightly disturbing, fact. While we happily interbreed with each other, producing a continuous spectrum of inter-races, we are reluctant to give up our divisive racial language. Wouldn't you expect that if all intermediates are on constant display, the urge to classify people as one or the other of two extremes would wither away, smothered by the absurdity of the attempt, which is continually manifested everywhere we look? But this is not what happens, and perhaps that very fact is revealing. People who are universally agreed by all Americans to be "black" may draw less than one eighth of their ancestry from Africa, and often have a light skin colour well within the normal range for people universally agreed to be "white."
Many people - probably more than most of us realise - have both black and white ancestors. If we are going to use colour terminology, many of us are presumably somewhere in between. Yet society insists on calling us one or the other. It is an example of what, in the "Salamander's Tale" chapter of my new book, I have called the "tyranny of the discontinuous mind." Americans are regularly asked to fill in forms in which they have to tick one of five boxes: Caucasian (whatever that means - it certainly doesn't mean from the Caucasus), African-American, Hispanic (whatever that means - it certainly does not mean, as the word seems to suggest, Spanish), Native American or Other. There are no boxes labelled "half and half." But the very idea of ticking boxes in this way is incompatible with the truth, which is that many, if not most, people are a complex mix of the offered categories and others. My inclination is irritably to refuse to tick any boxes, or to add my own box labelled "human."
In the particular case of "African-Americans," there is something culturally equivalent to genetic dominance in our use of language. When Mendel crossed wrinkled peas with smooth peas, all the first generation progeny were smooth. Smooth is "dominant," wrinkled is "recessive." The first generation progeny all had one smooth allele and one wrinkled, yet the peas themselves were indistinguishable from peas with no wrinkled genes. When a white man marries a black woman, the progeny are intermediate in colour and in most other characteristics. This is unlike the situation with peas. But we all know what society will call such children: "black." Blackness is not a true genetic dominant like smoothness in peas. But social perception of blackness behaves like a dominant. It is a cultural or memetic dominant.
If all humans were wiped out except for one local race, the great majority of the genetic variation in the human species would be preserved. This is not obvious and may be surprising to some people. If racial statements were as informative as most Victorians used to think, for example, you would need to preserve a spread of all the different races in order to preserve most of the variation in the human species. Yet this is not the case. It certainly would have surprised Victorian biologists who, with few exceptions, saw humanity through race-tinted spectacles. Their attitudes persisted into the 20th century. Hitler was unusual in gaining the power to turn racialist ideas into state policy. Many others had similar thoughts but lacked the power. HG Wells's vision of his New Republic (Anticipations, 1901) is a salutary reminder of how a leading British intellectual, regarded in his time as progressive, could say horrifying things only a century ago and scarcely be noticed doing so: "And how will the New Republic treat the inferior races? How will it deal with the black?... the yellow man?... the Jew?... those swarms of black, and brown, and dirty-white, and yellow people, who do not come into the new needs of efficiency? Well, the world is a world, and not a charitable institution, and I take it they will have to go... And the ethical system of these men of the New Republic, the ethical system which will dominate the world state, will be shaped primarily to favour the procreation of what is fine and efficient and beautiful in humanity - beautiful and strong bodies, clear and powerful minds... And the method that nature has followed hitherto in the shaping of the world, whereby weakness was prevented from propagating weakness... is death... The men of the New Republic... will have an ideal that will make the killing worth the while." I suppose we should take comfort from the change that has come over our attitudes during the intervening century. Perhaps, in a negative sense, Hitler can take some credit for this, since nobody wants to be caught saying anything that he said.
We were dealing with
the unusually high level of genetic uniformity in the human species, despite
superficial appearances. If you take blood and compare protein molecules,
or if you sequence genes themselves, you will find that there is less difference
between any two humans living anywhere in the world than there is between
two African chimpanzees. We can explain this human uniformity by guessing
that our ancestors, but not those of the chimpanzees, passed through a
genetic bottleneck, perhaps within the last 100,000 years. The population
was reduced to a small number, came close to going extinct, but pulled
through. Like the children of Noah in the myth, we are all descended from
this small population, and that is why we are so genetically uniform.
Some people find the evidence of biochemical genetics unsatisfying because it seems not to square with their everyday experience. We don't "look" uniform. Norwegians, Japanese and Zulus really do look rather dramatically different from one another... Nevertheless, we can't write off the genetic evidence which suggests that, all appearances to the contrary, we really are an unusually uniform species. What is the resolution to the apparent conflict between appearance and measured reality?
It is genuinely true that, if you measure the total variation in the human species and then partition it into a between-race component and a within-race component, the between-race component is a very small fraction of the total. Only a small admixture of extra variation distinguishes races from each other. That is all correct. What is not correct is the inference that race is therefore a meaningless concept... we can all happily agree that human racial classification is of no social value and is positively destructive of social and human relations... but however small the racial partition of the total variation may be, if such racial characteristics as there are are highly correlated with other racial characteristics, they are by definition informative, and therefore of taxonomic significance. "Informative" means something quite precise. An informative statement is one that tells you something you didn't know before. The information content of a statement is measured as reduction in prior uncertainty. If I tell you that Evelyn is male, you immediately know a whole lot of things about him... Your prior uncertainty about his ability to lift weights or excel at most sports is quantitatively reduced, but only quantitatively. Plenty of females can beat plenty of males at any sport, although the best males can normally beat the best females. Your ability to bet on Evelyn's running speed, say, or the power of his tennis serve, has been slightly raised by my telling you his sex, but it has not reached certainty.
Now to the question of race. If I tell you Suzy is Chinese, how much is your prior uncertainty reduced? You now are pretty certain that her hair is straight and black (or was black), that her eyes have an epicanthic fold, and one or two other things about her... It works the other way around to some extent. If I tell you Carl is an Olympic sprinting champion, your prior uncertainty about his "race" is, as a matter of statistical fact, reduced. Indeed, you can have a fairly confident bet that he is "black."
Suppose we took full-face photographs of 20 randomly chosen natives of each of the following countries: Japan, Uganda, Iceland, Sri Lanka, Papua New Guinea and Egypt. If we presented 120 people with all 120 photographs, my guess is that every single one of them would achieve 100 per cent success in sorting them into six different categories... yet an opposite prediction would seem to follow from his statement that racial classification has virtually no taxonomic or genetic significance.
We are indeed a very uniform species if you count the totality of genes, or if you take a truly random sample of genes, but perhaps there are special reasons for a disproportionate amount of variation in those very genes that make it easy for us to *notice* variation, and to distinguish our own kind from others. These would include the genes responsible for externally visible "labels" like skin colour. I want to suggest that this heightened discriminability has evolved by sexual selection, specifically in humans because we are such a culture-bound species. Because our mating decisions are so heavily influenced by cultural tradition, and because our cultures, and sometimes our religions, encourage us to discriminate against outsiders, especially in choosing mates, those superficial differences that helped our ancestors to prefer insiders over outsiders have been enhanced out of all proportion to the real genetic differences between us.
As with the insects
mating in the vicinity of their preferred food plants, people tend to mate
with others speaking the same language and praying to the same gods. So
different languages and religions can play the role of food plants, or
of mountain ranges in traditional geographical speciation. Different languages,
religions and social customs can serve as barriers to gene flow. From here,
according to the weak form of our theory, random genetic differences simply
accumulate on opposite sides of a language or religion barrier, just as
they might on opposite sides of a mountain range
- this is an edited version of The Grasshopper's Tale which appeared in "Prospect" magazine
RENDEZVOUS #27 -#39
During the Second World
War, sonar operators looking for submarines were puzzled by what seemed
to be a false bottom of the sea that rose towards the surface every evening,
and sank back down again the next morning. It turned out to be the bulk
of plankton, millions of tiny crustaceans and other creatures, rising to
feed near the surface at night, then sinking at morning.
- from The Jellyfish's Tale
All evolving creatures
track changes in the world: changes in the weather, in temperature, rainfall
and - more complicated because they hit back in evolutionary time - changes
in other evolving lines such as predators and prey. Some evolving creatures
alter, by their very presence, the world in which they live, and to which
they must adapt. The oxygen we breathe was not there before green plants
put it there. At first a poison, it provided radically changed conditions
that most animal lineages were forced first to tolerate, and then to thrive
- from The Polypifer's Tale
Whenever humans have
good idea, zoologists have grown accustomed to finding it anticipated in
the animal kingdom. Examples pervade this book, including echo-ranging
(bats), electrolocation (the Duckbill's Tale), the dam (the Beaver's Tale),
the parabolic reflector (limpets), the infrared heat-seeking senor (some
snakes), the hypodermic syringe (wasps, snakes and scorpions), the harpoon
(cnidarians) and jet propulsion (squids). Why not the wheel?
- from The Rhizobium's Tale
Future visitors from
outer space, who mount archaeological digs of our planet, will surely find
ways to distinguish designed machines such as planes and microphones, from
evolved machines such as bat wings and ears. It is an interesting exercise
to think about how they will make the distinction. They may face some tricky
judgements in the messy overlap between natural evolution and human design.
If the alien scientists can study living specimens, what will they make
of fragile, highly strung racehorses and greyhounds, of snuffling bulldogs
who can scarecly breathe, of walking udders such as Friesian cows, or walking
woolly jumpers such as Merino sheep? Molecular machines - nanotechnology
- crafted for human benefit on the same scale as bacterial flagellar motor,
may pose the alien scientists even harder problems.
- from The Rhizobium's Tale
We should be seeking
not the origin of life, which is vague and undefined, but the origin of
heredity - true heredity... five rivals breath as imagery for life. Our
ancestors who first tamed it probably thought fire a living thing, a god
even. Fire stays alive as long as you feed it. Fire breathes air; you can
suffocate it by cutting off its oxygen supply, you can drown it with water.
As with wolves, our ancestors could capture a fire cub as a useful pet,
tame it, feed it regularly and clear away its ashy excreta. Before the
art of firemaking was discovered, society would have prized the lesser
art of husbanding a captured fire.
Wild fires would have been observed giving birth to daughter fires... but still there was no true heredity. True heredity would mean the inheritance not of fire itself but of variations among fire. Blue fires don't beget blue fires. With dogs, like begets like. With fires, all the variation comes from the environment, none descends from the progenitive spark. Fires exhibit reproduction without heredity.
The origin of life was the origin of true heredity; we might even say the origin of the first gene - and by first gene I mean first replicator. A replicator is an entity, for example a molecule, that forms lineages of copies of itself. The key to true heredity is that each replicator resembles the one from which it was copied more than it resembles a random member of the population. The origin of the first such replicator was not a probable event, but it only had to happen once. Thereafter, its consequences were automatically self-sustaining and they eventually gave rise, by Darwinian evolution, to all of life.
THE HOST'S RETURN
If, as returning host, I reflect on the whole pilgrimage of which I have been a grateful part, my overwhelming reaction is one of amazement. Amazement not only at the extravaganza of details we have seen; amazement, too, at the very fact that there are any such details to be had at all, on any planet. The universe could so easily have remained lifeless and simple - just physics and chemistry, just the scattered dust of the cosmic explosion that gave birth to time and space... not only did evolution happen: it eventually led to beings capable of comprehending the process, and even of comprehending the process by which they comprehend it.
Evolution was never aimed at a particular endpoint. And yet, if, to borrow a thought experiment of the American biologist Stuart Kauffman, evolution could be rerun again and again - maybe on an imaginary sample of earthlike planets - how similar would the results be? The Kaufmann question is akin to the science fiction question of what life on other planets might be like - except that on other planets the starting and prevailing conditions would be different. On a large planet, gravity would impose a whole new set of selection pressures.
Like any zoologist, I can search my mental database of life on this planet and come up with an estimated answer to questions of the form: "How many times has X evolved independently?" "The" eye has evolved more than 40 times, to nine different "designs". Echolocation - the trick of emitting sound pulses and navigating by accurate timing of the echoes - has evolved at least four times: in bats, toothed whales, oilbirds and cave swiftlets. Not as many times as the eye's tally of 40-60, but still often enough to make us suspect that, if the conditions are right, sonar will evolve. To do the counts more systematically would make a good research project. What systems have evolved many times independently, like eyes? Or "several times", like echolocation? Have some things evolved only once, or not at all? I suspect that we'd find certain potential evolutionary pathways which life is "eager" to go down. Other pathways have more "resistance".
Elsewhere, I developed the analogy of a huge museum of all life, both real and conceivable, with corridors going off in many dimensions to represent evolutionary change, both real and conceivable. The corridor of eyes is wide open, almost beckoning. Other corridors are blocked off by barriers that are hard or even impossible to surmount. Evolution repeatedly races down the easy corridors, and just occasionally, and unexpectedly, leaps one of the hard barriers.
The venomous sting
(injecting poison hypodermically through a sharp-pointed tube) has evolved
at least 10 times independently: in jellyfish and their relatives, in spiders,
scorpions, centipedes, insects, molluscs (cone shells) snakes, the shark
group (stingrays), bony fish (stonefish), mammals (male platypus) and plants
(stinging nettles). It's a good bet that venom, including hypodermic injection,
would evolve in reruns.
Sound production for social purposes has evolved independently in birds, mammals, crickets and grasshoppers, cicadas, fish and frogs. Electrolocation, the use of weak electric fields for navigation, has evolved several times in fish and the duckbilled platypus. So has the - probably subsequent - use of electric currents as weapons. The physics of electricity is the same on all worlds, and we could bet with some confidence on repeated evolution of creatures that exploit electricity for both navigational and offensive purposes.
True flapping flight, as opposed to passive gliding or parachuting, has evolved four times: in insects, pterodactyls, bats and birds. Parachuting and gliding of various kinds evolved many times, maybe hundreds of times independently, and may be an evolutionary precursor to true flight. Examples include lizards, frogs, snakes, "flying" fish, squids, colugos, marsupials and rodents (twice). I'd put a lot of money on gliders turning up in hypothetical reruns of evolution, and a reasonable sum on true flapping fliers.
Jet propulsion may have evolved twice. Cephalopod molluscs do it, at high speed in the case of squids. The other example I can think of is also a mollusc, but it is not high-speed. Scallops mostly live on the sea bottom, but occasionally they swim. They rhythmically open and close their two shells, like a pair of snapping castanets. You'd think that this would propel them "backwards" in a direction opposite to the snapping. In fact, they move "forwards", as though biting their way into the water. How can this be? The answer is that the snapping movements pump water through a pair of apertures behind the hinge. These two jets propel the animal "forwards". The effect is so counter-intuitive it is almost comical.
But how about things that have evolved only once, or not at all? The wheel, with a true, freely rotating bearing, seems to have evolved only once, in bacteria, before being finally invented in human technology. Language, too, has apparently evolved only in us: that is to say at least 40 times less often than the eye. It is surprisingly hard to think of "good ideas" that have evolved only once.
The development of weapons, from the stone to the spear through the lonbow, the flintlock, musket, rifle, machine gun, shell, atomic bomb, through hydrogen bombs of increasing megatonnage, represents progress according to someone's value system, even if not yours or mine - otherwise the research and development to produce them would not have been done. Evolution exhibits progress not just in the weak, value-free sense. There are episodes of progress that are value-laden, according to at least some entirely plausible value systems. Since we are talking armaments, it is a good moment to note that the most familiar examples come out of arms races between predators and prey.
"Before asserting that
the deceptive appearance of a grasshopper or butterfly is unnecessarily
detailed, we must first ascertain what the powers of perception and discrimination
of the insects' natural enemies. Not to do so is like asserting that the
armour of a battle-cruiser is too heavy, or the range of her guns too great,
without inquiring into the nature and effectiveness of the enemy's armament.
The fact is that in the primeval struggle of the jungle, as in the refinements
of civilized warfare, we see in progress a great evolutionary armament
race - whose results, for defence, are manifested in such devices as speed,
alertness, armour, spinescence, burrowing habits, nocturnal habits, poisonous
secretions, nauseous taste, and procyptic, aposematic and mimetic coloration;
and for offence, in such counter-attributes as speed, surprise, ambush,
allurement, visual acuity, claws, teeth, stings, poison fangs, and anticryptic
and alluring coloration. Just as greater speed of the pursued has developed
in relation to increased speed of the pursuer; or defensive armour in relation
to aggressive weapons; so the perfection of concealing devices has evolved
in response to increased powers of perception."
- Hugh Cott, "Adaptive Coloration in Animals" (1940)
If it's amazement you want, the real world has it all... it is not pride in my book but reverence for life itself that encourages me to say, if you want a justification for the latter, open the former anywhere, at random.
BEYOND THE BOOK
"The book misses no
opportunity to take potshots at the Bush administration, which, funny as
they are, may soon make it seem very dated. In a book covering three billion
years it is odd to let 2004's obsessions into the text."
- Matt Ridley reviews "The Ancestor's Tale" in "The Guardian"
"There is not a scientist
writing today who expounds his subject for the lay reader with such scintillating
clarity and sheer politesse for the limits of the non-specialist... A biologist
colleague at Cambridge complains that, while it is admirable that every
candidate for admission to the biological sciences has read at least one
of Dawkins's books, the tragedy is that few of them have read anything
else. His grievance, I suspect, should be less about Dawkins than the failure
of his colleagues to write similarly readable studies."
- John Cornwell reviews "The Ancestor's Tale" in "The Sunday Times"
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