Evolution study tightens human-chimp connection

Scientists at the Georgia Institute of Technology have found genetic evidence that seems to support a controversial hypothesis that humans and chimpanzees may be more closely related to each other than chimps are to the other two species of great apes – gorillas and orangutans. They also found that humans evolved at a slower rate than apes.

Appearing in the January 23, 2006 issue of the Proceedings of the National Academy of Sciences, biologist Soojin Yi reports that the rate of human and chimp molecular evolution – changes that occur over time at the genetic level – is much slower than that of gorillas and orangutans, with the evolution of humans being the slowest of all.

As species branch off along evolutionary lines, important genetic traits, like the rate of molecular evolution also begin to diverge. They found that the speed of this molecular clock in humans and chimps is so similar, it suggests that certain human-specific traits, like generation time, began to evolve one million years ago - very recently in terms of evolution. The amount of time between parents and offspring is longer in humans than apes. Since a long generation time is closely correlated with the evolution of a big brain, it also suggests that developmental changes specific to humans may also have evolved very recently.

In a large-scale genetic analysis of approximately 63 million base pairs of DNA, the scientists studied the rate at which the base pairs that define the differences between species were incorrectly paired due to errors in the genetic encoding process, an occurrence known as substitution.

"For the first time, we've shown that the difference in the rate of molecular evolution between humans and chimpanzees is very small, but significant, suggesting that the evolution of human-specific life history traits is very recent," said Yi.

Most biologists believe that humans and chimpanzees had a common ancestor before the evolutionary lines diverged about 5-7 million years ago. According to the analysis, one million years ago the molecular clock in the line that became modern humans began to slow down. Today, the human molecular clock is only 3 percent slower than the molecular clock of the chimp, while it has slowed down 11 percent from the gorilla's molecular clock.

This slow down in the molecular clock correlates with a longer generation time because substitutions need to be passed to the next generation in order to have any lasting effect on the species,

"A long generation time is an important trait that separates humans from their evolutionary relatives," said Navin Elango, graduate student in the School of Biology and first author of the research paper. "We used to think that apes shared one generation time, but that's not true. There's a lot more variation. In our study, we found that the chimpanzee's generation time is a lot closer to that of humans than it is to other apes."

The results also confirm that there is very little difference in the alignable regions of the human and chimp genomes. Taken together, the study's findings suggest that humans and chimps are more closely related to each other than the chimps are to the other great apes.

"I think we can say that this study provides further support for the hypothesis that humans and chimpanzees should be in one genus, rather than two different genus' because we not only share extremely similar genomes, we share similar generation time," said Yi.

Even though the 63 million base pairs they studied is a large sample, it's still a small part of the genome, Yi said. "If we look at the whole genome, maybe it's a different story, but there is evidence in the fossil record that this change in generation time occurred very recently, so the genetic evidence and the fossil data seem to fit together quite well so far."

The complete title of Darwin's most famous work, often abbreviated to The Origin of Species, was The Origin. of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. As Koster notes about Darwin's view on race, he:

'never considered "the less civilized races" to be authentically human. For all his decent hatred of slavery, his writings reek with all kinds of contempt for "primitive" people. Racism was culturally conditioned into educated Victorians by such "scientific" parlor tricks as Morton's measuring of brainpans with BB shot to prove that Africans and Indians had small brains, and hence, had deficient minds and intellects. Meeting the simple Indians of Tierra del Fuego, Darwin wrote: "I could not have believed how wide was the difference between savage and civilized man; it is greater than between a wild and domesticated animal . . . Viewing such a man, one can hardly make oneself believe that they are fellow creatures and inhabitants of the same world."44

Darwin's belief that some races (such as blacks) were inferior to others became so widely accepted that, as Haller concluded: 'the subject of race inferiority was beyond critical reach in the late nineteenth century.45 Although Darwin opposed all forms of slavery, he did conclude that one of the strongest evidences for evolution was the existence of living 'primitive races' which he believed were evolutionarily between the 'civilized races of man' and the gorilla:

'At some future period, not very distant as measured by centuries, the civilized races of man will almost certainly exterminate, and replace, the savage races throughout the world. At the same time, the anthropomorphous apes. . . will no doubt be exterminated. The break between man and his nearest allies will then be wider, for it will intervene between man in a more civilized state, as we may hope, even than the Caucasian, and some ape as low as a baboon, instead of as now between the Negro or Australian and the gorilla. ... It has often been said ... that man can resist with impunity the greatest diversities of climate and other changes; but this is true only of the civilized races. Man in his wild condition seems to be in this respect almost as susceptible as his nearest allies, the anthropoid apes, which have never yet survived long, when removed from their native country.' 46

For the lurkers, this is what Darwin actually wrote, in a generation when most educated people regarded the races of man as different species:

The Descent of Man, Charles Darwin: Chapter 7 - On the Races of Man:

We will first consider the arguments which may be advanced in favour of classing the races of man as distinct species, and then the arguments on the other side.
[snip]
Our supposed naturalist having proceeded thus far in his investigation, would next enquire whether the races of men, when crossed, were in any degree sterile.
[snip]
We have now seen that a naturalist might feel himself fully justified in ranking the races of man as distinct species; for he has found that they are distinguished by many differences in structure and constitution, some being of importance. These differences have, also, remained nearly constant for very long periods of time.
[snip]
On the other side of the question, if our supposed naturalist were to enquire whether the forms of man keep distinct like ordinary species, when mingled together in large numbers in the same country, he would immediately discover that this was by no means the case.
[snip]
But the most weighty of all the arguments against treating the races of man as distinct species, is that they graduate into each other, independently in many cases, as far as we can judge, of their having intercrossed. Man has been studied more carefully than any other animal, and yet there is the greatest possible diversity amongst capable judges whether he should be classed as a single species or race, or as two (Virey), as three (Jacquinot), as four (Kant), five (Blumenbach), six (Buffon), seven (Hunter) ... or as sixty-three, according to Burke.* This diversity of judgment does not prove that the races ought not to be ranked as species, but it shews that they graduate into each other, and that it is hardly possible to discover clear distinctive characters between them.
[snip]
Some naturalists have lately employed the term "sub-species" to designate forms which possess many of the characteristics of true species, but which hardly deserve so high a rank. Now if we reflect on the weighty arguments above given, for raising the races of man to the dignity of species, and the insuperable difficulties on the other side in defining them, it seems that the term "sub-species" might here be used with propriety. But from long habit the term "race" will perhaps always be employed. The choice of terms is only so far important in that it is desirable to use, as far as possible, the same terms for the same degrees of difference.
[snip]
Those naturalists, on the other hand, who admit the principle of evolution, and this is now admitted by the majority of rising men, will feel no doubt that all the races of man are descended from a single primitive stock; whether or not they may think fit to designate the races as distinct species, for the sake of expressing their amount of difference.
[snip]
Now when naturalists observe a close agreement in numerous small details of habits, tastes, and dispositions between two or more domestic races, or between nearly-allied natural forms, they use this fact as an argument that they are descended from a common progenitor who was thus endowed; and consequently that all should be classed under the same species. The same argument may be applied with much force to the races of man.

The Descent of Man, by Charles Darwin: Chapter 21 - General Summary and Conclusion:

Through the means just specified, aided perhaps by others as yet undiscovered, man has been raised to his present state. But since he attained to the rank of manhood, he has diverged into distinct races, or as they may be more fitly called, sub-species. Some of these, such as the Negro and European, are so distinct that, if specimens had been brought to a naturalist without any further information, they would undoubtedly have been considered by him as good and true species. Nevertheless all the races agree in so many unimportant details of structure and in so many mental peculiarities that these can be accounted for only by inheritance from a common progenitor; and a progenitor thus characterised would probably deserve to rank as man.

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