Evolution of early
Homo
: An integrated biological perspective
Authors: Antón Susan C. , Potts Richard , Aiello Leslie C. 
Published: July 2014

Background

Until recently, the evolution of the genus
Homo
has been interpreted in the context of the onset of African aridity and the expansion of open grasslands.
Homo erectus
was considered to be a bona fide member of the genus
Homo
, but opinions diverged on the generic status of earlier, more fragmentary fossils traditionally attributed to
Homo habilis
and
Homo rudolfensis
. Arguments about generic status of these taxa rested on inferred similarities and differences in adaptive plateau. However, there was near-universal agreement that the open-country suite of features inferred for
Homo erectus
had evolved together and provided the adaptations for dispersal beyond Africa. These features foreshadowed those of more recent
Homo sapiens
and included large, linear bodies, elongated legs, large brain sizes, reduced sexual dimorphism, increased carnivory, and unique life history traits (e.g., extended ontogeny and longevity) as well as toolmaking and increased social cooperation.

Advances

Over the past decade, new fossil discoveries and new lines of interpretation have substantially altered this interpretation. New environmental data sets suggest that
Homo
evolved against a background of long periods of habitat unpredictability that were superimposed on the underlying aridity trend. New fossils support the presence of multiple groups of early
Homo
that overlap in body, brain, and tooth size and challenge the traditional interpretation of
H. habilis
and
H. rudolfensis
as representing small and large morphs, respectively. Because of a fragmentary and distorted type specimen for
H. habilis
two informal morphs are proposed, the 1813 group and the 1470 group, that are distinguished on the basis of facial anatomy but do not contain the same constituent fossils as the more formally designated species of early
Homo
. Furthermore, traits once thought to define early
Homo
, particularly
H. erectus
, did not arise as a single package. Some features once considered characteristic of
Homo
are found in
Australopithecus
(e.g., long hind limbs), whereas others do not occur until much later in time (e.g., narrow pelves and extended ontogeny). When integrated with our understanding of the biology of living humans and other mammals, the fossil and archaeological record of early
Homo
suggests that key factors to the success and expansion of the genus rested on dietary flexibility in unpredictable environments, which, along with cooperative breeding and flexibility in development, allowed range expansion and reduced mortality risks.

Outlook

Although more fossils and archaeological finds will continue to enhance our understanding of the evolution of early
Homo
, the comparative biology of mammals (including humans) will continue to provide valuable frameworks for the interpretation of existing material. This comparative context enables us to formulate and test robust models of the relationships between energetics, life history, brain and body size, diet, mortality, and resource variability and thereby yield a deeper understanding of human evolution.