Week of September 23, 2002
The early Paleozoic world:
the Cambrian explosion and
the significance of the
Burgess Shale.
This week you should:
|
Precambrian life was largely microbial but the evolution of specific prokaryotes
(like cyanobacteria) who released oxygen as they started using solar energy
and CO2 to build organic matter had tremendous repercussions
for later life forms. These aerobic bacteria oxygenated oceans, surface
waters and atmosphere. Most prokaryotes, however, are anaerobic (oxygen
is toxic to them). They did not all go extinct. Anaerobic microbes continue
to keep our planet habitable for aerobic forms of life by decomposing minerals
or fixing atmospheric nitrogen into compounds that aerobic organisms can
use. Anaerobic prokaryotes persist in micro-environments lacking
breathable oxygen, in soils and sediments but also in the bodies of aerobic
creatures.
Some prokaryotes appear to have formed a symbiosis (mutually advantageous association) to give rise to the first, larger eukaryotic cells. All eukaryotes are aerobic. The DNA from two parents can be combined through sexual reproduction and the increased possibilities of genetic recombinations from parents may be one reason that early eukaryotes diversified rapidly in many planktonic forms preserved as acritarchs. Some multicellular colonies probably gave rise to a division of labour among specialized individual cells. Some eukaryotic colonies developed a mode of reproduction where all the cells of a new individual developed from a single-cell embryo, giving rise to the new kingdoms of animals, fungi and plants. The Ediacaran biota is a community of organisms up to several centimeters in size. Mouthless and limbless, their nature remains uncertain. They may be the direct ancestors of today's animals or were perhaps an evolutionary "dead end". This community of organisms vanished towards the end of the Proterozoic eon, just as sedimentary rocks show increasing traces of new types of animals able to churn through the sediment. Algal stromatolites also declined during the same interval, probably because they were being grazed by new marine animals that were soft bodied and left no fossils. |
The fossilized shells of marine invertebrates which appear at the base of the Cambrian confirm that life was now thriving in seas worldwide. This sudden appearance of diverse mineralized fossils has traditionally marked the end of the Proterozoic eon and the beginning of the Phanerozoic eon. In the last two decades, the picture has become more complex. The Cambrian explosion took place in several waves but its most spectacular expression is the rapid diversification of animal body plans preserved at exceptional sites like the Burgess Shale of British Columbia.
What spurred this diversification? Is the appearance of hard parts, complex burrows and diverse body plans the result of a "biological arms race"? Or did this sudden acceleration of evolution follow a fundamental genetic change? We are not sure.
The waves of rapid diversification
and extinctions documented in early Paleozoic rocks show that evolution
is not simply a progressive increase in complexity. Environmental
changes probably played a crucial role in this early proliferation of marine
life, but the clues we gather from the rocks are incomplete. We do not
have a simple explanation for the choppy pace of evolution displayed by
Cambrian marine animal life.
| Keywords: |
The review questions relevant to this week's lectures are:
A few more questions to think about:
Useful links: