Chapter 17. The origins of life and Precambrian evolution

EVOLUTION/LECTURE1
10.- Early evidence of cells
10.1.- Once self-replicating systems evolved on the Earth, at least one of them adapted
to the use of DNA to store heritable information and to the use of proteins to express
that information. This system eventually gave rise to all lineages of life on the planet
today: We draw this conclusion because all life forms (except viruses) use DNA and
proteins. In fact, all modern organisms use them in the same way; the same
20 amino acids and the same basic structure of the genetic code have been found
in all creatures studied to date. Thus, we apply the principle of parsimony to infer
that all organisms share common ancestor
10.2.- Because another shared feature of all extant life is the existence of cells, we
also infer that the common ancestor was a cellular form. The advantages of
cellular membranes, as well as internal organellar membranes would have been
enormous. Cells allow for compartmentalization. This allowed life to accumulate
its necessary constituents in much higher concentration than they are found free
in solution. Cells allowed genotypes and phenotypes to be linked
10.3.- The first place we might look in trying to identify the ancestral cells is the fossil
record. The oldest fossils definitively established as those of living organisms are
3.465 billion years old (Fig. 14.15). These fossils come from Western Australia. they
show simple cells growing in short filaments
Figure 14.15.: The oldest known fossils of living organisms. The
fossils show filaments composed of individual cells lined up like
beads of a string
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