Scientists say a look at the right physical traits can be useful in research on human evolution
Monday, October 2nd 2000, 12:00 am
By: News On 6
By Emily Sohn / Special Contributor to The Dallas Morning News
Scientists searching for human ancestors may find skeletons to be deceiving.
Soft body tissues, though, confirm what genetic studies have long been saying â€“ that humans are more closely related to chimpanzees than either group is to other primates.
The idea that humans and chimps are more like siblings than cousins in the primate family initially shocked most people, who assumed that relationships mirrored physiques. Gorillas look like the chimpanzees they sit next to in the zoo. People assumed the cage bars drew hereditary lines as well.
But genetic studies in recent years have drawn a different primate family tree. Off the original line of primates, the tree shows, gibbons and siamangs branched first. Then orangutans diverged; then gorillas. Later â€“ some time between 5 million and 6 million years ago â€“ the human line split from the ancestors of today's chimpanzees, making chimps and humans the closest of primate relatives.
Yet when scientists compare hard tissues, such as bones and teeth, they sometimes find more similarities between chimps and gorillas than between chimps and people. Some scientists, therefore, doubt whether physical features can say anything about evolution at all. A new study, however, suggests that physical features may be useful â€“ as long as you look at the right ones.
A group of British and American scientists compared observations from studies describing 197 soft tissues â€“ such as muscles, blood vessels and nerves â€“ among five groups of primates. Those comparisons clearly supported the family tree, or phylogeny, drawn by genetics, the scientists reported last month in the Proceedings of the National Academy of Sciences.
"Lo and behold, if you look and see how these characters distribute, you do recover the molecular phylogeny," said Bernard Wood, one of the study's three authors.
One reason for the bone problem has been that two people can look at the same bone and describe its physical features in different ways, said paleoanthropologist David Pilbeam, of Harvard University in Cambridge, Mass. In the same way, a child's eye color might resemble his mother's, while eye shape might resemble his father's.
"The fact that eight different scholars ... come to five or six different answers tells you that you have a fundamental problem with breaking complex shapes down into characters," Dr. Pilbeam said.
Scientists tend to describe soft tissues in more consistent terms than hard tissues, he said. The study by Dr. Wood's group further avoided bias by using observations that were collected for reasons unrelated to the study, he said.
Looking at genes is still the most objective approach, though, and genetic studies have basically confirmed what the primate family tree looks like, Dr. Pilbeam said. Studies like this most recent one are now helping show which physical traits give the right answer, he said, like picking out which jigsaw puzzle pieces fit a given solution.
"The fact that we know what the correct tree is makes it possible to say this study is very interesting," Dr. Pilbeam said. "It's probably the first time somebody has been able to collect a large data set that gives a tree that is the correct tree."
More soft tissue evidence, combined with an accumulating understanding of genetics, may help scientists figure out better ways to look at bones and teeth, which make up the majority of fossils, said Dr. Wood, of George Washington University in Washington, D.C.
Genes work together to determine physical traits, and each trait is under the control of many genes. Researchers hope to eventually find links between genes, hard tissue features and soft tissue features.
"It shouldn't be all that long before we can join the two ends of the circle," Dr. Wood said.
The new finding also points to the importance of having lots of different kinds of studies supporting one another, especially in cases where relationships defy expectations, said anthropologist Adrienne Zihlman, of the University of California, Santa Cruz.
"You really need a fair amount of data to support what appears to be a counterintuitive or untenable position," she said.
The tree-dwelling koala is one example, she said. Its closest living relative is the wombat, a digging animal. Whales are most closely related to hippopotami, a fact few might guess. And giant pandas are closer to bears than to lesser pandas, Dr. Pilbeam said.
Dr. Wood said his work also emphasizes the need for more research on primates, whose populations are rapidly disappearing in many places.
"Now that we know that soft tissues are quite valuable, we need to get a move on," he said. "There are whole parts of the body for which we don't have information. Unless we get this information, these animals are going to go extinct."