264 HUMAN INTELLIGENCEH. naledi, it exhibits humanlike skeletal details but has a small brain,
although as yet it is undated). Th ese are the earliest known members of
the genus Homo; that is, the fi rst human species. Th ey lived in a period of
further forest thinning when traditional forest fruits must have started to
become scarce. Habilis sites are associated with a par tic u lar tool industry
characterized by crude stone fl akes, rounded hammer stones, and pos-
si ble stone weapons. Th ey are regularly found near dismembered animal
remains.
A review in Science in 2014 reported that “over the past de cade, new
fossil discoveries and... new environmental data sets suggest that Homo
evolved against a background of long periods of habitat unpredictability
that were superimposed on the under lying aridity trend.” It goes on,
“From ~2.5 to 1.5 million years ago, three lineages of early Homo evolved
in a context of habitat instability and fragmentation.... Th ese contexts
gave a selective advantage to traits, such as dietary fl exibility and larger
body size, that facilitated survival in shift ing environments.”^8 Th at pic-
ture is largely confi rmed in more recent reports. However, they still tend
to stress macro- environmental factors (dietary fl exibility, unpredictable
environments, range expansion, etc.) rather than cognitive demands of co-
operation per se.
Th e scatter of animal remains around Homo sites suggests living in
large groups with cooperative hunting, or possibly scavenging, as a habit.
But the most impor tant feature is an enlarged brain, in a cranium of
about 650 cubic centimeters. Th is is some 30 percent bigger than the Aus-
tralopithecine brains and is much bigger than that of chimpanzees (when
allowance is made for body size). In addition, they had lighter skeletons
and skulls, with dentitions much reduced in size, suggesting less reliance
on tough vegetable matter in the diet. All this suggests some parallel be-
tween cooperative hunting and living in larger social groups, on the one
hand, with brain size and cognitive complexity, on the other.
A new species, Homo erectus, fi rst appeared about 1.8 million years ago.
It is associated not only with another large increment in brain size but
also with continuing brain expansion across the period of its existence:
from 850 cubic centimeters at 1.5 million years ago to 1,200 cubic centi-
meters at 200,000 years ago (modern Homo sapiens has an average brain
size of 1,400 cubic centimeters). Homo erectus prob ably originated in
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