0 0.2 0.4 0.6 Divergence in DNA Sequence (percent)

0.6 0.4 0.2 0 Divergence in DNA Sequence (percent)

ancestors, whereas dead fossils may not have descendants. Molecular biologists know the genes they are examining must have been passed through lineages that survived to the present; paleontologists cannot be sure that the fossils they examine do not lead down an evolutionary blind alley.

The molecular approach is free from several other limitations of paleontology. It does not require well-dated fossils or tools from each part of the family tree it hopes to describe. It is not vitiated by doubts about whether tools found near fossil remains were in fact made and used by the population those remains represent. And it concerns itself with a set of characteristics that is complete and objective.

A genome, or full set of genes, is complete because it holds all the inherited biological information of an individual. Moreover, all the variants on it that appear within a population—a group of individuals who breed only with one an-other—can be studied, so specific peculiarities need not distort the interpretation of the data. Genomes are objective because they present evidence that has not been defined, at the outset, by any particular evolutionary model. Gene sequences are empirically verifiable and not shaped by theoretical prejudices.

The fossil record, on the other hand, is infamously spotty because a handful of surviving bones may not represent the majority of organisms that left no trace of themselves. Fossils cannot, in principle, be interpreted objectively: the physical characteristics by which they are classified necessarily reflect the models the paleontologists wish to test. If one classifies, say, a pelvis as human because it supported an upright posture, then one is presupposing that bipedalism distinguished early hominids from apes. Such reasoning tends to circularity. The paleontologist's perspective therefore contains a built-in bias that limits its power of observation.

As such, biologists trained in modern evolutionary theory must reject the notion that the fossils provide the most direct evidence of how human evolution actually proceeded. Fossils help to fill in the knowledge of how biological processes worked in the past, but they should not blind us to new lines of evidence or new interpretations of poorly understood and provisionally dated archaeological materials.

Molecular Clock

ALL THE ADVANTAGES of our field stood revealed in 1967, when Vincent M. Sarich, working in Wilson's labora tory at the University of California at Berkeley, challenged a fossil primate called Ramapithecus. Paleontologists had dated its fossils to about 25 million years ago. On the basis of the enamel thickness of the molars and other skeletal characteristics, they believed that Ramapithecus appeared after the divergence of the human and ape lineages and that it was directly ancestral to humans.

Sarich measured the evolutionary distance between humans and chimpanzees by studying their blood proteins, knowing the differences reflected mutations that have accumulated since the species diverged. (At the time, it was much easier to compare proteins for subtle differences than to compare the genetic sequences that encode the proteins.) To check that mutations had occurred equally fast in both lineages, he compared humans and chimpanzees against a reference species and found that all the genetic distances tallied.

Sarich now had a molecular clock; the next step was to calibrate it. He did so by calculating the mutation rate in other species whose divergences could be reliably dated from fossils. Finally, he applied the clock to the chimpanzee-human split, dating it to between five million and seven million years ago—far later than anyone had imagined.

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