How Evolving Traits Helped Humans Survive Unstable World

the skulls of three early human species, including Homo rudolfensis, Homo habilis and Homo erectus, — Three early human species likely co-existed at the dawn of humanity between 2.1 million and 1.8 million years ago, including the 1470 group (likely *Homo rudolfensis*) and the 1813 group, likely *Homo habilis*, (left and second from left, respectively). The other fossils represent *Homo erectus*, which evolved by 1.8 million to 1.9 million years ago.

(Image credit: Chip Clark, Smithsonian Human Origins Program (left three); Guram Bumbiashvili, Ge orgian National Museum (right))

Three different human species may have walked the Earth at the dawn of the human lineage, dividing up their environment in slightly different ways, and the ancestors of modern humans may have survived because oftraits such as large brains that helped them adapt to unstable, shifting landscapes, researchers say.

Moreover, the defining features of the human lineage may not have evolved together gradually at once, but piecemeal in stages over millions of years, scientists added.

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However, recent fossil evidence suggests these traits did not arise together as a single package. Instead, key human features evolved piecemeal at separate times, with some emerging substantially earlier and some later than previously thought. For instance, recent findings suggest long legs, a feature once considered unique to humans, developed in earlier ancestors, the genus Australopithecus, between 3 million and 4 million years ago, and stone tools about 2.6 million years old may predate the origin of Homo.

Scientists have long suggested that human evolution was linked to the onset of global cooling and the spread of a stable or progressively arid savanna grasslands in Africa. However, recent studies hint that early Homo may have evolved in a far more diverse environment, with the birthplace of humanity dominated from 2.5 million to 1.5 million years ago by an unstable climate, shifting intensity of annual wet and dry seasons, and varied landscapes.

"Unstable climate conditions favored the evolution of the roots of human flexibility in our ancestors," study co-author Richard Potts, a paleoanthropologist and curator of anthropology and director of the Human Origins Program at the Smithsonian's National Museum of Natural History, said in a statement. "The narrative of human evolution that arises from our analyses stresses the importance of adaptability to changing environments, rather than adaptation to any one environment, in the early success of the genus Homo."

"Taken together, these data suggest that species of early Homo were more flexible in their dietary choices than other species,"study co-author Leslie Aiello, a paleoanthropologist and president of the Wenner-Gren Foundation in New York,said in a statement. "Their flexible diet — probably containing meat — was aided by stone tool-assisted foraging that allowed our ancestors to exploit a range of resources."

These early human species include Homo erectus, the most likely ancestors of Homo sapiens. The researchers currently dub the other two species the 1470 group, traditionally classified as Homo rudolfensis, which had a relatively tall, flat face, and the 1813 group, traditionally classified as Homo habilis, which had a more primitive face, whose roof of the mouth was more rounded toward the back of the head. (The 1470 and 1813 groups get their names from the numbers assigned key fossils defining each lineage.)

Fossil skulls suggest human brains grew larger and more complex between 200,000 and 800,000 years ago. At the same time, the level of diversity and innovation of human tools increased, and signs of cooperative food-sharing are seen around hearths and shelters, developments that would help the human lineage survive uncertain environments. Ultimately, human flexibility was likely essential to Homo expanding out of Africa, with Homo erectus reaching what is now the nation of Georgia starting about 1.8 million years ago.

Fossils and archaeological finds dating between 2.5 million and 1.5 million years ago that scientists could discover in the future might help shed light on the evolution of early Homo. In the meantime, to learn more about human evolution, scientists can do more research on the biology of modern humans and other living animals. This will help develop and test models involving the intricate relationships between brain and body size, diet, mortality and other factors "to help us interpret the fossil and archaeological evidence that we now have," Aiello said.

Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.