The ultimate colonists: human ancestors settled into one ecosystem after another - Stone Age evolution

Science News, July 5, 2003 by Bruce Bower

The Stone Age was rough on community life, at least among animals trying to make a living in Africa. A range of species would move into a local habitat--gazelles, zebras, pigs, people, you name it--and take a few generations to establish the web of interactions that characterizes an ecosystem. After a millennium or so, dramatic climate shifts would then radically remodel the habitat, motivating the residents to leave. Eventually, a new collection of species would inhabit the area.

As these communities formed, dispersed, and reformed, one line of creatures always found a place in the mix--members of the Homo lineage, the ancestors of people today. Therein lies a couple of surprising lessons for researchers trying to untangle Stone Age evolution, says anthropologist Richard Potts of the Smithsonian Institution in Washington, D.C.

First, climate fluctuations reshaped regional habitats in unique ways rather than exerted uniform environmental effects worldwide or even across continents, as scientists have often assumed. As a result, animals had the option to leave hostile habitats and find greener pastures. Earlier theories have treated dramatic climate change primarily as a trigger for extinctions and the evolution of new species.

Second, although long envisioned as a time when our ancestors lived stable, simple lives, the heart of the Stone Age may actually have featured much social ferment and innovation, as people adjusted to shifting habitats throughout Africa. No brand of mammal, it seems, adapted to new ecosystems with the deftness of the genus Homo.

"We can see for the first time that strong environmental variability [during the Stone Age] was mirrored in the ecological domain and was powerful enough to disrupt any continuity in community evolution," Potts says. "Only [human ancestors] were adaptable enough to colonize the diverse animal communities that were repeatedly built up and broken down."

These climate fluctuations (SN: 7/12/97, p. 26), dramatic makeovers of local environments, and community transitions appear to have gone on throughout the Stone Age, says Potts. He presented his findings in Tempe, Ariz. at the recent annual meeting of the Paleoanthropology Society.

COME TOGETHER Potts' curiosity about prehistoric ecosystems had been piqued by a wave of scientific interest in how modern species come to coexist in specific areas. Much of this discussion focuses on three factors that influence the composition of species in an animal community: the existence of dominant competitors for food and other resources, the capacity of certain animals to move quickly and claim fertile new areas, and the overall fit of a species with the climate, available food, and other environmental variables.

Potts wondered whether the Stone Age fossil record reflected those three factors. He began by examining the site he knows best, Olorgesailie in southern Kenya.

Olorgesailie has yielded not only nonhuman animal bones but also stone tools and toolmaking debris in sediment layers that range from 500,000 to 1.2 million years old. Three fossil samples, each containing bones from a variety of mammals, proved crucial for Potts' analysis of this site. The first sample dates to 990,000 years ago and accumulated approximately 1,000 years. The second dates to 900,000 years ago and also covers a 1,000-year span. The third dates to 650,000 years ago and encompasses perhaps several thousand years.

Each sample held a different community of species, Potts notes. Wild horses are the most abundant animals in the oldest group of fossils. That title passes to large, baboon-like creatures--each about the size of a female gorilla--in the next set of fossils. Extinct forms of pigs and hippopotamuses predominate in the youngest sample.

Surprisingly, gazelles and other hoofed creatures, known collectively as bovids, accounted for only 13 percent to 28 percent of the mammals at each time. Bovids typically make up 60 percent to 80 percent of the mammal fossils found at East African fossil sites.

That anomaly concerned Potts. It raised the possibility that the repeated formation and breakup of animal communities at Olorgesailie was an isolated phenomenon.

His concern evaporated as he examined previously excavated animal remains from other African Stone Age sites ranging in age from 500,000 to 1.1 million years: Tanzania's Olduvai Gorge, a pair of Ethiopian sites in the fossil-rich Awash Valley, and sites in south-central and southern Africa.

The effort revealed that ancient animal communities contained distinctive blends of species from one part of Africa to another. Communities broke apart and reassembled in unique ways at Olduvai and in the south-central-Africa sample, just as at Olorgesallie. The Ethiopian and southern-Africa sites exhibited so much species change over time that Potts couldn't pin down clear community identities for them.

Strikingly, none of the factors that biologists currently have highlighted as determining the species composition of modern ecosystems appears to have loomed large at the African sites, Potts argues. No single, superior competitor species strutted its stuff at Olorgesallie or elsewhere throughout the Stone Age. Grassy expanses at all the Stone Age sites favored grazing animals, but a wide variety of these creatures came and went, with none achieving a lasting fit to the environment.