Pruning the family tree: a controversial study sends many hominid species packing.
In fact, the past 5 million years of hominid evolution witnessed the coming and going of many formerly unrecognized species, as the discoverers of new fossils in Africa now argue (SN: 8/19/95, p.119).
But the long-standing debate over how best to divvy up fragmentary fossils into hominid species has not vanished. Witness the contention of two South African researchers that they have statistically slashed through the thicket of proposed hominid species and left standing only the main stem of human evolution.
Hominids evolved gradually from one species to another, these researchers propose. Dispersed groups of each human ancestor interbred from time to time, adding layers of anatomical variety to a common biological theme. Fossil investigators often see two or more species where only one existed, they say, because they tot up minor skeletal differences that arose in biologically unitary hominids.
"I know it sounds outrageous, but we have good evidence that there was no more than one species at any one time in the hominid family--from a hypothetical ancestral ape to modern humans," contends Maciej Henneberg, an anatomist and paleontologist at the University of the Witwatersrand Medical School in Johannesburg. "Multiregional evolution goes back to the beginnings of the hominid lineage."
Henneberg and J. Francis Thackeray of the Transvaal Museum in Pretoria present their findings in the current Evolutionary Theory (vol. 16, no. 1).
Arguments for lumping diverse hominid remains into only a few species extend back at least 25 years. But scientific opinion currently favors expanding the number of hominid species (SN: 6/20/92, p.408).
Henneberg and Thackeray maintain that proposed hominid species living at roughly the same times displayed no more anatomical variation than our current species, Homo sapiens, in two crucial measurements--cranial capacity (an indirect marker of brain size) and body size.
To better understand the cranial variation in a sample of three ancient hominid skulls, the researchers randomly selected samples of three skulls from a worldwide collection of 10,000 modern H. sapiens skeletons. They repeated this sampling 1,000 times and calculated an acceptable range of variation. Henneberg and Thackeray then repeated this statistical exercise for sample sizes of 4 to 19 skulls.
They found, for example, that the extent of cranial variation in modern human samples met or exceeded the corresponding variation in 10 Australopithecus africanus skulls, 12 skulls from both A. africanus and A. afarensis, 13 skulls from early Homo and the robust australopithecines (considered a separate hominid genus by some scientists), 19 australopithecines from several commonly accepted species, 18 H. erectus skulls from Africa and Asia, and 10 Homo specimens from various regions dated at 1 million to 500,000 years old.
Any of these samples could therefore have come from a single species composed of separate populations bearing distinctive features, as occurs in modern humans, Henneberg maintains.
The range of body heights and weights in different sets of hominids that lived at about the same time also falls within the spread for modern humans, the South African researchers hold. Even fossils grouped by their ages rather than their species designations--such as all those from 4.5 million to 3.1 million years old or from 1.75 million to 1.25 million years old--differ no more in the extent of their body sizes than do modern humans.
The findings indicate that species have emerged over long periods in a single hominid lineage, according to Henneberg. "Rather than trying to arbitrarily decide where one species ends and another begins, we need to concentrate on studies of the processes of continuous evolution," he asserts.
Henneberg's opinion draws sharp criticism. For starters, counters Bernard Wood of the University of Liverpool in England, modern humans inhabit diverse environments around the world and thus display much more anatomical variation than any other species, past or present. For this reason, Wood doubts that H. sapiens provides a reliable anatomical yardstick for earlier hominid species that covered smaller portions of the globe.
Henneberg and Thackeray also fail to consider that specific clusters of anatomical features act as unique tags for hominid species, in Wood's opinion. Consider A. robustus, he says, which had large, peg-shaped teeth and a bony crest atop its skull that together distinguished it from Homo species of its time.
"We haven't recognized enough early hominid species," Wood argues. Ian Tattersall of the American Museum of Natural History in New York City agrees. Subtle body differences that do not show up on bones may often have distinguished hominid species from one another, he says.
Even Milford H. Wolpoff of the University of Michigan in Ann Arbor, who wants to merge H. erectus into a single species of H. sapiens that evolved over the past 2 million years, suspects that earlier hominids belonged to numerous species. Any group of animals that migrates to a new locale typically evolves survival-enhancing traits that gradually result in a new species, Wolpoff holds. In the unique case of H. sapiens, however, survival has increasingly hinged on culturally acquired knowledge and tools; thus people have settled in all corners of the world without evolving into separate species.
Alan G. Thorne of Australian National University in Canberra supports Wolpoff's view of H. sapiens, but he also welcomes Henneberg and Thackeray's findings. Only the robust australopithecines--which died out about 1 million years ago--show clear anatomical signs of having branched off the main stem of hominid evolution, Thorne holds.
Conclusions such as that of the South African scientists undermine arguments that some hominids, such as Asian H. erectus and Neandertals, did not contribute to human evolution, according to Thorne. Instead, anatomically distinct races capable of interbreeding have evolved over at least the past 2 million years, in his view.
"This new study provides a powerful argument in favor of a single, hardly branching hominid lineage," he remarks. "We'll have to start looking at teeth and other skeletal features to test this theory further."
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||research indicates only one hominid species existed at a time|
|Article Type:||Cover Story|
|Date:||Sep 2, 1995|
|Previous Article:||Got them low-fat, polyunsaturated blues.|
|Next Article:||A new twist on outside in.|