The theory that growing bigger was an important part of humanity's early evolution looks shaky. The first members of the genus Homo were smaller than previously thought, argues Dr. Mark Grabowski of George Washington University, and this forces a rethink of the adaptations that put our ancestors on the road to becoming us.
Some time around 2.8 million years ago, the australopithecine hominids evolved into Homo habilis, the first representatives of our genus. They were distinguished by their more advanced tool use and larger braincase. It has been previously thought that H. habilis and other early humans were also larger than australopithecines.
However, in the Journal of Human Evolution, Grabowski and co-authors report that the “last major review of hominin body size” was conducted two decades ago and since then “new fossils have been discovered, species attributions have been clarified and methods improved.”
Curious as we may be about the size of our ancenstors, there is another reason to explore changes in body size. “An animal's overall body size is directly related to how it interacts with the natural world. Factors such as energy requirements, home-range size, social organization, relative brain size, locomotion, and numerous other morphological, ecological, and life history characteristics are all tied in some way to body size,” the paper points out. “Thus, interpreting the evolution of any of these factors demands accurate estimates of body size in extinct species. This is true for our own lineage, where almost all of the hows and whys of human evolution are directly tied to estimates of body size at particular points in time.”
The authors used the weights of 220 modern humans to create equations for predicting body masses based on various bones. These were applied to a selection of fossils whose allocation to specific species is considered reliable.
“Our results show that many early hominins were generally smaller-bodied than previously thought, an outcome likely due to larger estimates in previous studies resulting from the use of large-bodied modern human reference samples,” the authors report.
The paper also provides an interesting insight into the way science can sometimes go wrong. In 1992, Henry McHenry conducted a similar study, but described it as an “important first step toward establishing the average body size and range of variation of early hominid species,” in light of a number of uncertainties. However, the more recent paper notes, “despite McHenry’s caveats about uncertainty regarding some of the estimates, the results of this study have often been used with more confidence than might be warranted.”
The idea that our origins as humans coincided with an increase in meat consumption and associated size has lingered.
With the expanded sample size and more advanced taxonomy, Grabowski says the team, “Found no evidence that the earliest members of our genus differed in body mass from earlier australopiths (some of the earliest species of hominins). In other words, the factors that set our lineage apart from our earlier ancestors were unrelated to an increase in body size, which has been the linchpin of numerous adaptive hypotheses on the origins of our genus."