The Riddled Chain by Jeffrey McKee, Rutgers University Press, $27,
ISBN 081352783X
SCIENCE is full of cowdung, that irreverent shorthand for 鈥渢he conventional
wisdom of the dominant group鈥�. Cowdung for us palaeontologists studying human
evolution is the notion that changes in climate favour some hominid species
while wiping others out. But Jeffrey McKee begs to differ.
In The Riddled Chain, he says human evolution occurs independently
of climate. The agent of change is a sort of internal evolutionary motor.
Evolution is self-propelled, or autocatalytic. Environment may shape evolution,
but it cannot catalyse it.
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If anyone can sell the principle of evolutionary autocatalysis as the driving
force of human evolution then it should be McKee. He writes well, using
nonspecialist language sprinkled with well-chosen and apt metaphors that make
arcane arguments accessible. He is fair to opposing arguments, and humble in a
good way. He comes across as the antithesis of the self-absorbed scientific
superstar. My own prejudice is that you need much more than internal mechanisms
to explain the coincidences that pervade the evolution of animals large and
small, mammal and insectivore, during the Plio-Pleistocene in Africa between 5
and 2 million years ago.
Still, I spent a long time mentally trying on autocatalysis to see if I could
feel comfortable with it. My main difficulty is that it has no predictive value.
McKee faces that argument head-on by saying that autocatalysis is unpredictable
because of its nonlinear nature. He dismisses the patterns other
palaeontologists perceive in evolutionary events as artefacts born out of the
vagaries of erosion and deposition.
In The Riddled Chain he roundly rejects the notion of a link between
climate and human evolution. He has good credentials: he cut his teeth doing
fieldwork at the celebrated fossil site of Taung in South Africa, where the
first African human ancestor was discovered. Since then he has taken a
commendably independent line with his research. He concentrates on computer
simulations to test whether peaks and troughs of evolutionary activity would
occur in any case because of the unpredictable, nonlinear, and thus chaotic,
nature of the evolutionary process. He claims that if he programs his computer
with just the restraints imposed by the patchy fossil record, the model
reproduces peaks and troughs of extinction and speciation. These are the same
peaks and troughs that others have cited as evidence for the 鈥渃limatic forcing鈥�
of evolution. So can the cowdung survive this challenge?
I think so. An unsung scientific achievement of the past century is the
recovery of a detailed record of past climates. One of the keys that unlocked
the door was the discovery by Harold Urey in 1946 that mollusc shells faithfully
record the ratio of light to heavy oxygen in seawater. The warmer the water, the
greater the rate of evaporation. The greater the rate of evaporation, the higher
the concentration of heavy oxygen in the shells, for the light form predominates
in the water vapour.
Two other developments in dating gave us the final clues
to past climates. Potassium-argon dating in rocks also depends on isotopic
ratios, while magnetostratigraphy reflects major shifts in the Earth鈥檚 magnetic
polarity.
The dating game
This handful of dating methods let scientists calibrate plankton samples in
deep-sea cores from the ocean floor. They could track changes in global
temperature over time using Urey鈥檚 oxygen isotope ratios as a proxy for seawater
temperature.
Meanwhile, in the same half-century, another group of researchers had been
making remarkable progress in documenting human evolutionary history. Fossil
discoveries, and complementary advances in the analysis of that fossil evidence,
forced them to reassess the shape of the human evolutionary tree.
Simple scenarios in which an ape-like ancestor evolved through a series of
intermediate stages, each progressively more like modern humans, had to be
abandoned in favour of schemes that were more like bushes than ladders.
Compelling evidence proved that at several stages in human evolution two, three
and perhaps as many as four types of primitive human had shared the East African
landscape. Experts even disagreed about how many creatures they were willing to
recognise in the fossil record. This made it far more difficult to link
ancestors with descendants鈥攁nd more contentious.
There is, however, fairly widespread agreement that the time between 3 and 2
million years ago was crucial for human evolution. After that, the human fossil
record in East Africa seems to have polarised into two lineages. The one linked
with our own genus Homo becomes increasingly like modern humans, while
the other one retains its distinctive morphology鈥攋umbo-sized faces and
jaws with chewing teeth to match鈥攗ntil its demise sometime before 1
million years ago.
The time that our two ancestral strands were becoming more distinct
corresponds with a major shift in global and regional climates. In East Africa a
warm, moist climate gave way to cooler and drier weather. Our ancestors were not
the only animal group to show marked changes. Antelopes, pigs and elephants, for
example, also seemed to be evolving rapidly.
Palaeoclimatologists and palaeontologists began to compare notes. One result
was Elisabeth Vrba鈥檚 elegant turnover-pulse hypothesis that sought to explain
how climate change could have influenced mammalian evolution in general, and
human evolution in particular.
There was good evidence that climate was affecting habitat, and Vrba proposed
that the effect on animal communities was predictable. The more specialised the
animals, the more immediate and drastic would be the effect of climate change.
Basically, you adapted if you could鈥攐r you became extinct. Vrba鈥檚
hypothesis suggested that the less-specialised animals could more effectively
roll with the climatic punches. They would be less likely to become extinct, or
speciate, and would be more likely to adapt.
This model seemed to fit the data鈥攐r rather, it did for a while. Then
researchers began to try to understand how gaps in the fossil record might have
biased our estimates of the first and last appearances of whole orders of
creatures.
If you run out of the right type of sediment in a rock layer, there will be
no fossil record for the next few hundred thousand years. All your taxa will
look as if they died out at the same time. In reality, while extinction dates
might have been accurate for some species, others could have been around for
hundreds of thousands of years longer. With no suitable strata to preserve them,
they would simply leave no trace of their continuing presence. McKee shows the
huge role chance plays in palaeontology.
My colleagues, and those who follow the twists and turns of human
evolutionary research, should read his book and make up their own minds. McKee鈥檚
message about the role of chance in making patterns is a sound one, but I am a
pattern addict. It will take more than The Riddled Chain, the
intellectual equivalent of a cold shower, to discourage me from seeking
connections among the evolutionary events of the African Plio-Pleistocene.