Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 137

Group of China one can find Leiosphaeridia , a rather dull-looking
spherical fossil, but one with a complex double structure to its organic
wall. Although unspectacular as fossils, the structures they produced
indicate the presence of the subcellular building blocks needed to
make, ultimately, a much wider array of shapes and structures. Despite
their greater cellular complexity, evolution proceeded slowly without
the possibility of complex multicellular structures. Then came the
invention of sex.
Bangiomorpha doesn't look like much. Indeed, you might easily
overlook this fossil organism as some tiny obscure strands of some-
thing or other in the rock. But if you have the precise eye, acute brain,
and gently eccentric manner of Nick Butterfield, a geologist at the
University of Cambridge, then Bangiomorpha is a very significant
organism indeed. 81 For a start, it is very old, occurring in rocks of the
Hunting Formation in Arctic Canada that are perhaps as old as 1.2
billion years: these rocks appear to be have been deposited in very
shallow water on the margins of a sea. Bangiomorpha shows different
types of cell (Fig. 12). It has structures designed to make a multicellu-
lar holdfast for attachment to a firm substrate. From this holdfast
arise unbranched filaments composed of multiple cells, and the lay-
ered arrangement of the cells in these filaments is directly comparable
to that of the modern red alga Bangia . Bangiomorpha also shows the
earliest evidence for sex, with at least two distinct spore-producing
phases.
The co-occurrence of sexual reproduction on complex multicellu-
lar organisms with specialized cell types may not be coincidental: one
may have led inevitably to the other. In the late Proterozoic the diver-
sity of eukaryotic organisms, both unicellular and multicellular,
increased dramatically. Sex, it seems, provided a rich source of evolu-
tionary novelty that would lead to large, three-dimensional organisms
populating the oceans.