Geoscience Reference
In-Depth Information
the energy-providing mitochondria, and the chloroplasts—all are
derived from originally independent organisms that went into part-
nership. Between them and their hosts they produced a new cellular
machine: a eukaryotic organism.
For geologists this can be a little depressing, because there is little
likelihood of finding fossil evidence of quite how this important event
happened, although some molecular 'fossil' evidence remains in the
genes of modern microbes. Whatever the unobserved, original mech-
anisms of this process, it clearly led to the ability to construct cells
and ultimately bodies with a much wider array of forms and func-
tions. Evidence of when this step took place can be found, though,
within preserved strata laid down on the floor of the Proterozoic
oceans.
Prokaryotic cells such as bacteria can be large. They can have proc-
esses that stick out from the cell, and they can have cell structures that
preserve as fossils. But no single prokaryotic cell possesses all three of
these characteristics, and neither do they possess the complex surface
architecture of eukaryotes. 80 For such complexity to develop requires
the invention of 'scaffolding': the development of a cytoskeleton. It
also requires subdivision by membranes within the cell ('endomem-
branes'). Both of these structures are characteristic of eukaryotic cells.
Based on these pragmatic criteria, the first appearance of eukaryotes
is seen in fossils from rocks in China and Australia from about 1.7
billion years ago.
In outward appearance, these early eukaryotic organisms are not
terribly spectacular. The earliest of them are considered to be acri-
tarchs, a group long considered to be the resting (spore) stages of phy-
toplankton, although in the Precambrian these fossils may cover a
wider biological range, possibly including animals. In the Mallapun-
yah Formation of northern Australia there are fossils of Valeria with
its distinctive 'corduroy' pattern of parallel lines, and in the Changcheng
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