Week of September 17, 2001
The Proterozoic eon: the buildup of atmospheric oxygen and the rise of animal life.
This week you should:
| The Earth is probably around 4.5 Ga old, but rocks of that age were eroded from its surface, or buried and strongly metamorphosed or re-melted. Only mineral grains (zircon) dated at 4.03-3.96 Ga have been found. Our planet became "fit for life" relatively quickly, possibly 500 million years after the time it formed and differentiated into a core, mantle and crust. The early atmosphere and oceans were practically devoid of molecular oxygen (O2), yet energy sources, water and organic molecules were available to sustain early life forms. Water may have accumulated by degassing of the mantle, but could also have been brought from space by icy comets. Comets and meteorites are known to carry organic compounds such as amino acids and they could have delivered them to Earth's surface. Sedimentary rocks preserve evidence of liquid surface water because characteristic structures are produced by the settling of particles. Sedimentary rocks are interlayered with igneous rocks that can be dated by radiometric methods. The oldest sedimentary rocks, now metamorphosed to gneisses found in Greenland, are 3.8 Ga old. Several hypotheses have been proposed about the sites where the earliest life might have evolved. Ecosystems supported by chemosynthetic bacteria living today at mid-ocean ridges show that life can be sustained without solar energy, deep in water that would have shielded it from harmful ultraviolet radiation. |
The Proterozoic eon is a huge slice of time (2 billion years!) marked by the progressive rise of oxygen in the Earth's atmosphere. How do rocks record changes that took place billions of years ago in the atmosphere? Where did the oxygen we breathe come from? What effect did its increase in oceans and in the atmosphere have on the evolution of life?
Darwin was rather irritated
by the fact that fossils appear so suddenly and in diverse forms, in rocks
of Cambrian age (544 Ma). This did not fit well with his suggestion that
life evolved slowly and in minute steps. Surely, the fossil record must
be incomplete. By 1946s, impressions of centimeter-size Precambrian life
forms were discovered in Australia, in rocks that are about 580 Ma-old.
More have been found since on other continents. But can we tell if they
were algae, microbes or animals? Did they behave like animals? Are they
plausible ancestors to today's life forms, or were they an early evolutionary
experiment?
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Review questions (on page
339) that are relevant to this week's lectures: 2, 3, 4, 5, 10, 11.