in the Pliocene
the Pliocene, the Panamanian land bridge linked North and South America,
allowing terrestrial species to migrate between the
two continents. This event is called the Great American
Faunal Interchange, a time when two long-isolated faunas came
into contact. Sixteen native southern genera moved to the north, including
armadillos, giant ground sloths, flightless predatory birds, marsupials
(including opossums), and porcupines. At the same time, 23 native
northern genera moved south, including cats, dogs, bears, tapirs,
camels, and certain rodents. The exchange was not
a balanced one, however. Many more South American immigrant species
became extinct, perhaps as a result of competition
and the inability to adapt to new conditions. As a
result, many North American species now live in South America, but
few South American imports still survive in the north.
Migrations also continued between Asia and North America
via the Bering land bridge. Mastodons and true horses (Equus)
were among these migrating mammals. Rodents appear to have reached
Australia from the Indonesian archipelago at this time. The one-toed
horse Equus was common in North America, along with armored
glyptodonts and many species of camels. The once-common
oreodonts had dwindled in number, and North American
rhinos became extinct.
Also during the Pliocene, several early bipedal ancestors
of humans co-existed in the African landscape. Early
evidence of human ancestors comes from East African fossil localities
in the rift valley sites of Ethiopia, Kenya, and Tanzania.
The well-known fossil "Lucy" was discovered in 1974 in Hadar, Ethiopia.
This member of the hominid lineage, named Australopithecus afarensis,
was found in sediments dating to over 3 million years ago. Meanwhile,
at Laetoli, Tanzania, human-like footprints were left by two individuals
in a volcanic deposit over 3 million years ago. These discoveries
indicate that human bipedalism must be even older, and new evidence
suggests it may be as old as 6 to 7 million years.