The ultimate colonists: human ancestors settled into one ecosystem after another.

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.

The animals that succeeded must have moved in quickly to inhabit a region abandoned by others, Potts says. However, in his view, mobility alone can't explain how such a wide variety of "lead grazers" paraded through the same sites.

Enter a fourth factor that Potts dubs environmental dynamics. This influence plays out as a kind of thrust-and-parry duel between animals and their shifting surroundings. The action proceeds as follows: A distinctive combination of species colonizes a region, coexists for a while, and departs in the face of massive environmental change. A new species mix better suited to the remodeled surroundings then fills the vacuum. During the Stone Age, this back-and-forth battle ensured that no single large-mammal community evolved as a continuous entity.

That's an intriguing possibility, remarks Ian Tattersall of the American Museum of Natural History in New York. Potts' ideas aren't yet confirmed and get scant attention among anthropologists. Nevertheless, they raise the possibility that our ancestors nimbly adjusted to a constantly changing world, according to Tattersall.

"The repeated assembly and disassembly of species communities apparently took place widely over Africa during the middle Stone Age," Potts says.

TOOLING UP There's one bit of consistency in this ancient picture Of volatile animal interactions. Stone tools have turned up in each collection of animal remains studied so far.

"[Early] toolmakers were already equipped by the middle Stone Age to accommodate to substantial diversity in their ecological settings," Potts asserts.

Paleontologists typically don't grant so much flexibility to our ancestors of that time--whether classified as Homo sapiens or as tiny of several other Homo species. Researchers have often portrayed the middle Stone Age, which runs from around 1.5 million to 400,000 years ago, as a time during which our ancestors stuck to a fairly narrow repertoire of behaviors. This argument hinges on the extended distribution of a single type of tool, the Acheulian hand axe. Scads of these double-edged, teardrop-shape implements have been excavated at African sites from throughout the middle Stone Age.

In a world of revolving-door ecosystems, Acheulian hand axes may well have been supremely adaptable instruments. These tools, which also appeared in China by around 800,000 years ago (SN: 3/4/00, p. 148), probably functioned as Stone Age versions of Swiss Army Knives, Potts theorizes. The sharp, easily graspable devices could be used in all sorts of environments to cut meat off carcasses, to dig up plants and tubers, and to whittle points on digging sticks, just for starters.

Many researchers who study ancient hand axes now regard them as having been used for a variety of purposes, notes archeologist Ofer Bar-Yosef of Harvard University. Nonetheless, ancient human groups in some regions died out as water sources dried up, he argues, whereas those at sites with consistent water supplies, such as Olorgesailie, managed to survive.

Potts plans to examine animal remains at Stone Age sites in China for signs of repeated formation and dispersal of distinctive animal communities. Evidence of considerable movement by human ancestors along a north-south corridor in eastern China already exists, he says.

CHANGE OR DIE In Potts' view, constant species shuffling in Stone Age ecosystems deals a bad hand to another influential idea, the turnover-pulse hypothesis. Devised by Elisabeth S. Vrba of Yale University, this theory holds that old species die out and new ones arise in response to massive changes in their physical surroundings.

In particular, according to Vrba, a global cooling trend around 2.5 million years ago led to the evolution of new types of rodents and antelopes and the extinction of ancient forms of pigs, monkeys, and giraffes.

Yet fossil evidence that appears to show the emergence of some species and the loss of others at a given location may actually reflect the dispersal and regrouping of animal communities, Potts contends.

New investigations support this possibility, he notes. Several studies presented at the recent annual meeting of the American Association of Physical Anthropologist in Tempe indicated that regional rises and falls in the populations of certain species occurred with surprising frequency in the Stone Age.

For instance, the relative abundance of species from three major bovid groups changed over time in unique ways at two sites in Ethiopia and two in Kenya, say Rene Bobe of the Smithsonian Institution and his coworkers. The dramatic thinning out of specific bovid species and the rise of others occurred at these sites as many as four times between 4 million and 1 million years ago, the scientists say. Changes in bovid populations are a sensitive marker of environmental change, Bobe notes.

Sites in the same part of Africa also witnessed local population shifts among monkey species, reports Stephen R. Frost of the American Museum of Natural History. One such shift transpired at Ethiopian and Kenyan sites around 3.4 million years ago, Frost says.

Subsequent shifts took place from 2.9 million to 2.5 million years ago in Ethiopia and 2 million years ago in Kenya. The 2.5-million-year-old cooling trend central to Vrba's turnover-pulse theory appears to have had little effect on the fate of monkey species in eastern Africa, Frost adds.

Such data point to the need for a closer analysis of how a variety of mammals fared at different points in the Stone Age, according to Potts. His provocative theory coincides with a growing interest among evolutionary biologists in how all sorts of organisms adapt to fluctuating environments.

Several mechanisms have been proposed for coping with environmental change. Lauren A. Meyers and James J. Bull, both of the University of Texas at Austin, described them in the December 2002 Trends in Ecology and Evolution.

One possibility is that environmental shifts induce major genetic changes in organisms. In an example of this phenomenon, published in the May 30 Science, Ivana Bjedov of Paris (France) University and her colleagues report that most strains of wild bacteria exhibit elevated DNA-mutation rates during periods of starvation. The genetic alterations that result aid survival when nutrients are scarce, the scientists say.

Another prospect is for a population to maintain different lineages, each with its own environmental specialty. Consider a species of desert flower in which a white-flowered variety fares well in wet years but a blue-flowered version does well in dry years.

Alternatively, organisms can produce offspring so diverse that at least some can weather drastic environmental changes.

Moreover, the evolution of extended individual development, as in H. sapiens, may have stoked an aptitude for learning and innovation that permits human adaptation to one habitat after another (SN: 9/21/02, p. 186).

Still, much about Stone Age evolution remains unexplained, Potts acknowledges. "For years, I and many others simply connected the dots from one ancient animal community to the next through time, treating them as part of a continuous evolutionary process," Potts says. "We can't do that anymore."