Human ancestors make evolutionary changes.
Some scientists believe that Homo erectus, the species directly ancestral to modern humans, is a model of evolutionary stability and a prime example of the theory of "punctuated equilibrium,' which holds that individual species have a clear beginning and end (SN: 7/25/81, p. 52). This view was fostered by a recent study indicating that for nearly 1.5 million years these precursors of Homo sapiens remained largely the same, rapidly changing in form and developing larger brains only when a new species was about to appear.
Several lower forms of life are undoubtedly marked by long periods of relatively little change followed by rapid transformations into new species, a cornerstone of punctuated equilibrium theory, but this pattern clearly does not apply to H. erectus, contends an anthropologist who recently trekked throughout the world to survey all known H. erectus specimens. "It appears that there are significant evolutionary changes within a conservatively defined sample of H. erectus,' says Milford H. Wolpoff of the University of Michigan in Ann Arbor.
Wolpoff, whose research itinerary included stops in China, Indonesia and North America, took a variety of skull, jaw and dental measurements from 92 of these "prehumans.' He divided the specimens, which date from about 1.4 million years old to 400,000 years old, into early, middle and late H. erectus groups. Averages of the measurements for each group were compared across the 1-million-year span.
With only a few exceptions, he found pronounced differences between the early and late H. erectus samples. The changes are in the direction of a modern profile, reports Wolpoff in the justreleased Fall 1984 PALEOBIOLOGY; cranial capacity expands while jaw and tooth size shrinks. The few skull and jaw features that remain stable do not detract from the evidence that two major "adaptive systems' of the H. erectus lineage changed substantially over time, he says.
Wolpoff also studied 13 individuals who are either late H. erectus or early H. sapiens. There is no distinct boundary between the two species, he says, again suggesting that in this case punctuated equilibrium theory does not apply. That theory, as proposed in 1977 by Stephen J. Gould of Harvard University and Niles Eldredge of the American Museum of Natural History in New York City, assumes that there are clear demarcations between successive related species, and that evolutionary changes are often spontaneous responses to unexpected environmental demands. The continuous, although not necessarily constant, rates of change within H. erectus do not reflect this assumption, adds Wolpoff.
Gould and Eldredge first used H. erectus as an example of their theory following a 1981 report by G. P. Rightmire of the State University of New York at Binghamton. He studied 65 individuals designated as H. erectus and concluded that the species did not significantly evolve over time.
But Rightmire's study is seriously flawed, says Wolpoff. Up to 16 of the specimens he used may not be H. erectus, and his statistical analysis was not adequate to uncover evolutionary changes.
Rightmire acknowledged to SCIENCE NEWS that he would take a different statistical approach if he conducted a new study. "But it's difficult to see [Wolpoff's study] as a coherent statement on the entire species,' he argues. "Wolpoff is hardly following a conservative approach to defining H. erectus.'
When two specimens that may not be H. erectus are taken out of Wolpoff's early sample and another is removed from the late sample, there is no statistically meaningful difference between the cranial capacities of the two groups, says Rightmire. "There are signs of rapid evolutionary change, especially in brain size, as Homo erectus gave way to Homo sapiens, although this does not necessarily mean there was a branching of species as punctuated equilibrium theory predicts.'
"[Rightmire] is absolutely wrong,' responds Wolpoff. There is no justification for removing the three specimens from the study, he says, but even without them the sample is large enough to be unaffected by the loss of a few individuals.
"The more interesting issue now is to examine the speed and direction of evolutionary change in Homo erectus,' says Wolpoff.
Adds Philip D. Gingerich, director of the Museum of Paleontology at the University of Michigan: "I think Wolpoff was quite conservative in his definitions of which specimens are Homo erectus. We always want more details, but his study is a step above anything that has been done before.'
Photo: Skull of a 1.6 million year old H. erectus youth, discovered after Wolpoff's survey.