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CHAPTER 15 • Humans and Preindustrial Climate 275

probably spent most of their time in trees, gathering
fruits and nuts and avoiding predators, but they also
moved on the ground when necessary.
A remarkable deposit dated to 3.6 Myr ago in Tanza-
nia holds footprints of two creatures walking across
a freshly fallen layer of volcanic ash that had cooled
(Figure 15–4). The tracks show that one of the creatures
turned, perhaps to look back at something, and then
walked on. These human-like apes of the genus Australo-
pithecus (known as australopithecines) walked upright to
travel. Scientists argue whether these creatures devel-
oped the ability to walk in order to exploit food resources
on the grassy savanna lying between stands of trees, or
whether they developed upright postures in order to
stand on the lower tree limbs (or on the ground) and
reach up for fruits and nuts.
Use of Stone Tools The first firm evidence that
hominins used stone tools dates to about 2.5 Myr ago.
These early tools, used to butcher dead animals, pro-
duced marks on the animal bones that indicate crushing
and scratching but are sometimes difficult to distinguish
from similar marks made by the teeth of carnivores
(lions, leopards, cheetahs).
One early hypothesis suggested that humans
evolved mainly as “killer apes” because of the aggressive
use of tools to kill their prey. But many anthropologists
today believe that hominins simply made opportunistic
use of the remains of animals previously killed by lions
FIGURE 15-3 Early mammals The line of early mammals
from which humans evolved included creatures
resembling these modern-day lemurs. (Frans Lanting/
Minden Pictures.)

A new source of evidence—molecular biology—
reduced this uncertainty. Molecular biologists measure
the composition of DNA molecules in the protein of
living organisms. They postulate that DNA works like
an evolutionary clock: the longer the time that has
elapsed since two organisms branched off from a com-
mon ancestor, the more dissimilar their DNA will have
become. If this DNA dissimilarity increases through
time at a constant rate, the degree of dissimilarity can
be used as a clock to measure elapsed time. Molecular
biologists concluded that the line that led to humans
diverged from the line that led to the modern great apes
between 6 and 4 Myr ago (see Figure 15–2).
Walking Upright The evolutionary line that led
to modern humans, called hominins (from the family
Hominidae, meaning human-like), appeared by 4 Myr
ago. Fossil remains of anklebones with a distinctive
structure suggest that walking had become the primary FIGURE 15-4 Footprints from 3.6 Myr ago Hominin
means of movement by 4.3 Myr ago. These creatures (humanlike) creatures that walked across fresh volcanic ash
were considerably smaller than modern humans and 3.6 Myr ago in East Africa left their footprints, now fossilized.
had chimpanzee-like faces with large, strong jaws. They (Kenneth Garrett, National Geographic Society Image Collection.)