though they may still have used trees as refuges. They were shorter than
humans, the tallest being less than ¿ ve feet tall. At about 400–500 cubic
centimeters (cc), their brains were slightly larger than those of chimps (300–
400 cc) but about one-third the size of human brains (which average about
1,350 cc). Some may have made stone tools, but there is no sign of signi¿ cant
technological ability or enhanced linguistic ability. Though we are probably
descended from one branch of the australopithecines, we would probably not
think of them as humans if we were to meet one in the street today.
Homo habilis (“handy man”) appeared about 2.3 million years ago. Homo
habilis were probably just under 5 feet tall and had brains of 600–800 cc.
Louis Leakey regarded them as the ¿ rst real humans because they made
stone tools. (This is a powerful reminder of how paleontologists can shape
such stories. Leakey belonged to a generation that regarded the use of tools
as the key marker of humanity.) Tool use implies considerable intellectual
ability and may have improved diets by making it easier to scavenge meat.
But today few paleontologists share Leakey’s view that they were the ¿ rst
true humans. This is partly because they had small brains, partly because
other ape species have been shown to use tools, and partly because their
technologies did not evolve signi¿ cantly.
Another species, Homo ergaster, evolved about 1.8 million years ago. They
were as tall as us, and their brains (at about 1,000 cc) were almost as large
as ours. They made more sophisticated “Acheulian” stone tools, often in the
form of carefully manufactured stone “axes.” Members of this species were
the ¿ rst hominines to migrate out of Africa. As these migrants entered the
colder lands of southern Eurasia, reaching as far as modern Beijing, they
may have learned to use ¿ re. Nevertheless, their stone tools changed little
over 1 million years, which suggests that they too lacked the technological
creativity that distinguishes modern humans.
By 1 million years ago most of the features that de¿ ne us as a species were
already present, but there was no sign yet of the technological and ecological
creativity that makes us so different. Clearly some sort of threshold still had
to be crossed. But before we describe the crossing of that critical threshold,
we must examine the evidence used to trace the evolution of our species. Ŷ