Magazine article Science News

Out on a Limb

Magazine article Science News

Out on a Limb

Article excerpt

The science of body development may make kindling out of evolutionary trees

Over the past 25 years, paleoanthropologists have nurtured one evolutionary tree after another, hoping to reap ever sturdier portrayals of humanity's descent. Each of these trees sprouts out of an approximately 5-million-year-old presumed common ancestor and then branches along lines for various australopithecine and Homo species, ending at Homo sapiens. Recently, investigators have become fond of assigning new fossil finds to unique rather than established species, so evolutionary trees have gotten downright bushy.

When paleoanthropologists craft evolutionary trees, they usually select bone and tooth measurements, sometimes as many as several hundred from different fossils, to plug into a computerized statistical analysis. This technique, called cladistics, places species on related limbs of the tree according to the number of recently evolved skeletal characteristics that they share with each other but with no other species. Scientists also have applied cladistics to differences in the nucelotide sequence of segments of DNA.

However, a growing number of investigators, including some formerly ardent evolutionary-tree nurturers now suspect that the branching cladistic creations suffer from conceptual root rot. The whole enterprise rests on shaky biological and misleading statistical assumptions, they say.

Anthropologists first need to enter the unfamiliar territory of developmental biology and learn how complex chains of molecular processes form the bones and soft tissues that coalesce into bodies, argues Tim D. White of the University of California, Berkeley.

As a codiscoverer more than 25 years ago of the famous 3.2-million-year-old remains of the human ancestor now known as Lucy, White has spawned his share of evolutionary trees. Over the past decade, though, advances in developmental biology have led him to abstain from that practice.

Biological research now suggests that as an organism grows from fetus to adult, the shape and arrangement of its bones reflect a cascade of molecular processes that were ignited by certain genes but then responded flexibly to a variety of influences in their biological milieu. So, genuinely informative skeletal traits for evolutionary trees are relatively uncommon and can't be identified by simply eyeballing specimens, he says.

"You can't [break up] skeletal anatomy, put all of the traits into a [statistical] program, and generate something that makes biological sense," White asserts. "A lot of people who have published cladistic trees are going to be in trouble."

White may be right, but statistical comparisons of our fossil ancestors' skeletal features are likely to remain popular, remarks anthropologist Erik Trinkaus of Washington University in St. Louis. Many researchers prefer to explore evolution with cladistic analyses of bones, DNA, or both, he says.

Agreement on the general technique doesn't imply agreement on the best data to use with it. Scientists frequently disagree about which skeletal traits are recently evolved and which specimens share them.

Skull and tooth measurements from people, chimps, gorillas, and orangutans--the same measurements frequently employed to generate evolutionary trees for human ancestors--give rise to a primate evolutionary tree that clashes with the genetic version, reported Bernard Wood of George Washington University in Washington, D.C., and Mark Collard of University College, London, in the April 25 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. These genetic data have set an evolutionary standard that skeletal traits can't match, Wood and Collard concluded.

Genetic studies have converged on the view that people bear a particularly close evolutionary relationship to chimpanzees, while gorillas and orangutans arose along separate evolutionary lines, according to these researchers. …

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