A new study published online March 26 in the Journal of Human Evolution suggests that the genus Homo has come in different sizes since its origins over 2 million years ago, and adds weight to the hypothesis that early human ancestors began to colonize Eurasia while still small and lightweight.
An artist’s reconstruction of a female Homo, probably Homo erectus, from Dmanisi, Georgia. Image credit: Elisabeth Daynes, via tabula.ge.
The authors of the study, Dr Jay Stock from the University of Cambridge, UK, and Manuel Will of Tübingen University, compare the physiology of individuals from early human populations, using fossils from the Olduvai Gorge in Tanzania, Koobi Fora in Kenya, as well as individuals from the Cradle of Humankind in South Africa. The sample also included fossils of early Homo from Dmanisi, Georgia.
The scientists found differences in body sizes which strongly indicate that there were at least two differently sized species of Homo living between 2.5 and 1.5 million years ago (Pleistocene).
According to the team, some individuals from Kenya’s Koobi Fora region (1.7-1.5 million years ago) would have stood almost 6 feet (1.78 m) high and weighed up to 71 kg, whereas their contemporaries from Tanzania’s Olduvai averaged 4.8 feet (1.47 m) tall and weighed 50 kg.
“These findings have important implications for the evolution of our genus,” Manuel Will said.
One of the dominant hypotheses of our evolution is that genus Homo evolved from small-bodied early human ancestors to become the taller, heavier and longer legged Homo erectus that was able to migrate beyond Africa and colonize Eurasia. However, the timing and geographic origin of the larger body size has remained unknown, until now.
Estimated heights and weights of early Homo during Pleistocene. Image credit: University of Cambridge.
The main increase in body size, according to the new study, occurred tens of thousands of years after Homo erectus left Africa, and primarily in the Koobi Fora.
“Before 1.7 million years ago our ancestors were seldom over 5 feet tall or particularly heavy in body mass,” Dr Stock said.
“When significant size shift to much heavier, taller individuals happened, it occurred primarily in one particular place, Kenya’s Koobi Fora, around 1.7 million years ago. That means we can now start thinking about what regional conditions drove the emergence of this diversity, rather than seeing body size as a fixed and fundamental characteristic of a species.”
He added: “it’s possible to interpret our findings as showing that there were either multiple species of early human, such as Homo habilis, Homo ergaster and Homo rudolfensis, or one highly diverse species.”
“This fits well with recent cranial evidence for tremendous diversity among early members of the genus Homo.”
“If someone asked you ‘are modern humans 6 foot tall and 70 kg’ you’d say ‘well some are, but many people aren’t,’ and what we’re starting to show is that this diversification happened really early in human evolution,” Dr Stock said.
_____
Manuel Will & Jay T. Stock. Spatial and temporal variation of body size among early Homo. Journal of Human Evolution, published online March 26, 2015; doi: 10.1016/j.jhevol.2015.02.009
