Dec. 25 is one day when many people make an effort to be with their families, and some might even take time to remember those less fortunate than themselves. So in that spirit, in this week’s column we’re going to think about our closest relatives, ones who are far less fortunate than us and who face complete extinction in, at best, 50 years.
A study earlier this year found that these close relatives, chimpanzees, share 99.4 percent of their genes with us. This means that humans and chimpanzees (and bonobo chimpanzees) are more similar to each other than they are to the other great apes — gorillas and orangutans. Chimps are close enough relatives that they should be allowed around the table this Christmas lunchtime, metaphorically speaking: They should be taken care of like other members of our family.
Derek Wildman (and could there be a better name for someone researching human/chimp evolution?) led the group that carried out the work, at the Wayne State University School of Medicine, Detroit. The team concluded that chimps should be reclassified into the Homo genus. We are Homo sapiens, so chimps would become Homo troglodytes (they are currently Pan troglodytes).
Wildman and colleagues compared 97 key genes from chimps, humans, bonobos, gorillas, Old World monkeys and, for good measure, mice. The differences in DNA sequence showed that humans and chimpanzees separated from gorillas between 6 and 7 million years ago. The most recent common ancestor of humans and chimpanzees lived between 5 and 6 million years ago.
This is by no means the final word on human-chimp genetic similarity. Earlier this month the chimp genome project published a rough draft. Many of the 3.1 billion bases of chimp DNA have now been sequenced, and soon a more thorough comparison will be made between the genomes of chimps and humans.
University of Tokyo professor Yoshiyuki Sakaki is the leader of the international consortium decoding the chimp genome and also project director of the Genomic Sciences Center at RIKEN in Yokohama. Only a chimp-human comparison, he said, “will show what makes humans human.”
But what does it mean to be human? Some characteristics of humans — language, ability to have abstract thought — might not, after all, be restricted to humans. Chimps have been taught American sign language, can recognize themselves in a mirror and can even enjoy watching others on television. Should we say that chimps are 99.4 percent human? Mice share 60 percent of our genes, but it seems ridiculous to say they are 60 percent human. Where do we draw the line?
Figures showing such a close similarity can be confusing. If so many of the two species’ genes are the same, then how come we are so different? The answer is that it is not so much which genes each animal has, but how they are regulated. How genes are turned on and off during our development is critical to how we turn out.
So, although we might have many genes in common, the precise DNA sequences of those genes are often different. It’s probably the reason why, despite the close similarity, chimps infected with HIV don’t develop AIDS.
Scientists have already identified many key sequence differences between human and chimp genes. In the journal Science this month, a team led by Michele Cargill from Celera Diagnostics in Alameda, Calif., found that humans’ genes for olfaction, hearing and speech (and others), had evolved more rapidly than the equivalent genes in the chimpanzee.
The team compared 7,600 genes shared by humans, chimps and mice, and looked for those that had evolved more rapidly than the baseline mutation rate. Faster than basic mutation means that the genetic change probably endowed a survival advantage, and was preserved by natural selection.
Human genes thought necessary for speech development (“speech is considered to be a defining characteristic of humans,” wrote the authors) had undergone particularly fast change, whereas in chimps genes controlling skeletal development had changed more than expected. Several human genes involved in the development of hearing were also identified as having undergone positive selection, and Cargill and her team speculate that our development of spoken language may have required tuning of hearing acuity.
The development of our sense of smell is more puzzling given that we don’t rely on it as we presumably did 5 million years ago in Africa. Or do we? Perhaps our behavior toward the opposite sex is informed by olfactory signals more than we know: Perhaps sexual selection drove the change in our smelling genes.
Where does this leave us in the quest to understand what it means to be human? I don’t know, but surely one thing that humans can uniquely do is act altruistically, independently of the direction of natural selection. We can help others, even if they are not family members. Strangers. Africans who now survive by trading in bushmeat. Yet there is no getting away from the fact that we are letting our closest relatives become extinct before our eyes.