Comment: 03:55 - 04:59 (01:04)
Source: Annenberg/CPB Resources - Earth Revealed - 11. Evolution Through Time
Keywords: fossil, "marine organism", bone, shell, stromatolite, algae, oxygen, atmosphere, evolution, "sedimentary rock"
Our transcription: Therefore, the record of life that does survive from the geologic past is skewed toward marine organisms.
Of these, organisms with bones or shells are most likely to be fossilized.
Only rarely do the softer body parts leave fossil impressions, and so almost all ancient organisms lacking hard parts have faded into obscurity.
There are, however, a few spectacular exceptions.
One of these is stromatolites.
These lumpy algal mats were the dominant life form throughout much of Earth's history but are now restricted to a few small areas of warm, shallow water.
Stromatolitic algae added immense amounts of oxygen to Earth's early atmosphere with important consequences for the evolution of life.
Fossil stromatolites appear as pillar-shaped layers of sedimentary debris.
The debris was trapped in the hair-like filaments of the algae when it was still alive. Algal tissue itself is not preserved.
Geology School Keywords