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Thursday, Jan. 13, 2005
Fossils reveal human drift to 'beauty'
By ROWAN HOOPER
The 18th-century British philosopher David Hume said "Beauty is no quality in things themselves: It exists merely in the mind which contemplates them; and each mind perceives a different beauty."
With time, this became condensed into today's familiar soundbite: "Beauty is in the eye of the beholder."
But is it?
When asked, people from cultures from all over the world will agree on who is beautiful and who is not, regardless of their genetic background. Such agreement contradicts Hume.
Clearly, not everyone looks as good as the actor Takeshi Kaneshiro or the actress Scarlett Johansson. So it seems likely that the "beauty is in the eye of the beholder" phrase arose to provide consolation to those without film-star looks.
Most of us are average, just look around you in the office or on the train to work. Most people do not look movie-star beautiful, though everyone (identical twins excepted) does look immediately and distinctively different from everyone else. OK, we might not all be beautiful, but why do we all look so different? How did such diversity in looks evolve?
Natural selection is the traditional answer. But it is not the only one, as archaeologists discovered at the end of last year.
Rebecca Ackermann of the University of Cape Town, South Africa, and James Cheverud of Washington University School of Medicine, St. Louis, MO, measured facial features of the skulls of early humans and our ancestors. The scientists found that some characteristics of early human facial structure first developed as the result of natural selection from about 2 to 3 million years ago. This gave our ancestors "robust" faces with heavy brows and prominent jaws (see photo).
But things changed from about 1 to 2 million years ago, when our genus of apes, Homo, evolved. When this branch split away, the remainder retained their burly looks, while Homo developed a petite skull.
At that time it wasn't just natural selection that was at work, say Ackermann and Cheverud. It was random genetic drift. This is a process whereby genes are fixed in a population by chance alone, not because they provide a selective advantage. It is especially common in small populations.
"While natural selection may select for or against some mutation, diversity which is produced through genetic drift has no adaptive advantage," said Ackermann in a recent e-mail interview.
To test whether genetic drift or natural selection had influenced our faces, the scientists made facial measurements on fossil skulls from seven early hominids and early members of our genus, such as Homo habilis and Homo erectus. They measured facial landmarks easily found on all specimens. For comparison, they also measured the face features on skulls of adult humans, gorillas and chimpanzees.
Ackermann and Cheverud assessed the pattern of variation both within and between the groups. They found that within the genus Homo there was a random pattern of variation among facial features. This suggests that genetic drift played more of a role than natural selection.
"Cultural inheritance could have released many of the morphological traits of humans from the pressures of stabilizing selection," Ackermann and Cheverud wrote, in Proceedings of the National Academy of Sciences.
What this means is that as human culture developed, we became sheltered from the raw strength of natural selection. We developed technology. At first, just stone tools, which were used to kill animals and chop them up. And when we started using fire to cook the meat, there was another relaxing effect on natural selection. Paleontologists tell us that both advances appear in the fossil record at about the same time as the genus Homo.
With these advances we no longer needed such a huge jaw to eat our dinner. The skull didn't need the heavy ridges that the jaw muscles attached to, and we didn't need the large canines to tear the meat. Our skulls and faces became gracile, as anthropologists put it.
And with the relaxing of natural selection, facial diversity was free to increase. Hence the vast differences we see today.
Faces also became another way to attract females. Our distant ancestors fancied men with hulking great jaws, but our more recent ancestors preferred good-looking guys. Attractiveness in a man became as important as aggressiveness. Some females preferred it.
Evidence for this theory was also published last year, this time by Eleanor Weston, of the Research Institute Senckenberg in Frankfurt, Germany. In the journal Biology Letters, Weston and colleagues showed that in species where females choose their mates, such as chimpanzees, the canines of males are roughly the same size as those of females. However, where males fight for females, such as baboons and gorillas, the canines are far larger.
The end point (or the current point) of this process can be seen in today's movie stars.
Johnny Depp, Leonardo DiCaprio, Orlando Bloom as well as the aforementioned Takeshi Kaneshiro, each have quite "feminized" faces.
Why are they popular? Because female preferences have evolved to prefer such men. Why? Because large eyes, broad cheek bones, clear skin and a symmetrical face signal good health, good genes and a good immune system. In a literal sense, then, beauty is in the eye of the beholder. We have been selected to appreciate beauty. For that we can thank natural selection. For the diversity of faces in the world, we can blame genetic drift.
A book of Natural Selections columns translated into Japanese, "Nou to sekkusu no seibutsugaku (Evolution, Sex and the Brain)," is published by Shinchosha. Rowan Hooper is a biologist at Trinity College, Dublin. He welcomes readers' comments at email@example.com