years ago. This mass extinction may have been caused by an asteroid impact,
though at present this is by no means certain. Another possibility is that it
was caused by the coming together of many once-separate regions to form
a supercontinent within which many different species had to compete for
fewer niches. The removal of so many earlier species created space for a
rapid “adaptive radiation” of new species. Two important new groups of
vertebrates were the dinosaurs and mammals, both of which appeared in
the Triassic period, between 250 and 210 million years ago. The class of
mammals contains furry, warm-blooded organisms that nurture their young
within their mother’s body and feed them with milk. (Even humans have fur,
though not much!) The earliest mammals were probably small, shrew-like
animals that foraged for insects at night.
About 67 million years ago, right at the end of the Cretaceous period, an
asteroid impact (the “Cretaceous event”) destroyed most large species,
including dinosaurs. That there was such an impact was demonstrated only in
the 1980s by geologist Walter Alvarez. Mammal species diversi¿ ed rapidly
in a new adaptive radiation, ¿ lling niches vacated by the dinosaurs. As part
of this mammalian radiation there appeared a new order of mammals, the
primates: tree-dwelling mammals with stereoscopic vision, hands designed
to grasp, and larger brains. We’ll cozy up to the primates in the next lecture
because we, too, are primates.
We have seen four crucial stages in the evolution of multi-celled organisms
like ourselves. Each of the eight stages of evolution described in this and the
last lecture contributed something to the makeup of our own species, Homo
sapiens. We will see how in the next lecture. Ŷ
Brown, Big History, chap. 2.
Christian, Maps of Time, chap. 5.
Fortey, Life: An Unauthorised Biography.
Essential Reading