Gone are the days when paleoanthropologists could characterize the path of human evolution as a simple, single homogenous line of progressive changes in human morphology and behavior. Or so suggests a collaborative group of scientists who, as detailed in a study published in the July 4, 2014 issue of Science, came up with a new synthesis, or at least the rudimentary framework of one.
It is a developing scenario that, they argue, more accurately explains how earlier forms of Homo (early humans) and their Australopithecus forerunners eventually led to the emergence of Homo sapiens (modern humans), the last surviving hominin.
Led jointly by Susan Antón, professor of anthropology at New York University, paleoanthropologist Richard Potts, curator of anthropology and director of the Human Origins Program at the Smithsonian’s National Museum of Natural History, and Leslie Aiello, president of the Wenner-Gren Foundation for Anthropological Research, the team studied paleoclimate, fossil, and stone tool evidence, leading to a developing consensus that suggests a rethinking of long-held assumptions about human origins and evolution.
Based on a synthesis of the data, the researchers point to change and diversity of environmental conditions and the compelling need to survive by adapting to the changing conditions as a key to understanding how early humans were able to vary, survive and begin spreading from Africa to Eurasia 1.85 million years ago.
The process entailed a diversification of species and genera differentiating and overlapping in time and morphology, beginning with some key elements once thought to define Homo but actually evolving in earlier Australopithecus ancestors between 3 and 4 million years ago.
Many traits unique to humans were long thought to have originated in the genus Homo between 2.4 and 1.8 million years ago in Africa. A large brain, long legs and the ability to craft tools along with prolonged maturation periods were all thought to have evolved together at the start of the Homo lineage as African grasslands expanded and Earth’s climate became cooler and drier. Now a paper published in Science today outlines a new theory that the traits that have allowed humans to adapt and thrive in a variety of varying climate conditions evolved in Africa in a piecemeal fashion and at separate times.
“Unstable climate conditions favored the evolution of the roots of human flexibility in our ancestors,” said Potts. “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.”
Their analysis and conclusions could explain, at least in part, much of the new evidence that has emerged in recent years suggesting that multiple coexisting species of Homo overlapped geographically and developed differentiating morphological and behavioral characteristics. It contrasts with the long-held model of a large brain, long legs, the ability to craft tools and prolonged maturation periods evolving together as a single package at the start of the Homo lineage as African grasslands expanded and Earth’s climate became cooler and drier.
The researchers also analyzed ancient stone tools, isotopes found in teeth and cut marks found on animal bones in East Africa.
“The traits that typify our own species Homo sapiens weren’t there right at the beginning of the evolution of the Homo genus; instead, humanness evolved in much more of a mosaic pattern,” explains Potts, curator of anthropology and director of the Human Origins Program at the Smithsonian’s National Museum of Natural History.
To reach these conclusions, the team took an innovative research approach, including developing a new climate framework based on the Earth’s astronomical cycles from 2.5 million to 1.5 million years ago. This paleoclimatic data was integrated with new fossils and understandings of the genus Homo, archaeological remains and biological studies of a wide range of mammals (including humans). However, it was the recently discovered skeletons of Australopithecus sediba (~1.98 Ma) from Malapa, South Africa, that really cemented the idea for Potts that the evolution of the Homo genus involved a period of evolutionary experimentation and mixing of traits.
“Taken together, these data suggest that species of early Homo were more flexible in their dietary choices than other species,” said Aiello. “Their flexible diet probably containing meat was aided by stone tool-assisted foraging that allowed our ancestors to exploit a range of resources.”
The study authors concluded that flexibility likely strengthened the ability of human ancestors to successfully adapt to changing environments and emerge out of Africa, and explains the ability of the modern human species to occupy diverse habitats throughout the world.
The detailed study is published in the July 4, 2014 issue of Science magazine.