Lecture 17: Life on Earth—Multi-celled Organisms
The second major transition on the road to modern humans is the
appearance of the ¿ rst vertebrates. We belong to the phylum of chordates,
or vertebrates—organisms with backbones. The ¿ rst vertebrates evolved
about 500 million years ago, in the Ordovician period, from worm-like
ancestors. They were probably unimpressive ¿ sh-like creatures with no heart
or brain, a bit like modern “lancelets.” All vertebrates have a front and back
end, and a complex system of internal communications through nerve cells
running along the spine. The vertebrates include ¿ sh, amphibia, reptiles,
birds, and mammals.The third major transition leading to humans is the movement of some multi-
cellular organisms from the sea to the land. Plants and insects probably
reached the land ¿ rst about 500 million years ago, during the Ordovician
period. The ¿ rst vertebrates to leave the sea did so about 400 million years
ago, during the Devonian period. Leaving the sea posed huge challenges.
You needed a strong skeleton as water no longer supported your weight.
You needed a tough skin to avoid drying out. You needed special apparatus
enabling you to breathe oxygen directly rather than through the water. And
you needed some way of reproducing in a watery environment so your
offspring would not dry out. The ¿ rst vertebrates to live for prolonged
periods on land were probably a bit like modern lung¿ sh, which can survive
for some periods on land if the ponds they live in dry out. Two important
“classes” of vertebrates were:x The amphibia, which lived on the land permanently but returned
to the water to lay eggs, and which evolved during the Devonian
period; andx The reptiles, which evolved about 350 million years ago, during the
Carboniferous period, and laid their young in eggs protected within
tough skins.The fourth transition on the path toward modern humans is the appearance
of the class of mammals about 250 million years ago, during the Triassic
period. As the supercontinent of Pangaea was forming, the majority of living
species vanished during the Permian period, from about 290 to 250 million