Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 141

to another, and were therefore an essential prerequisite to building
more complex ecosystems. In clearing the water of decaying organic
particles they have also been suggested as a means of helping the
spread of oxygen through the oceans. 85 One could predict, therefore,
that for complex marine ecosystems to develop on other planets some
kind of 'sponge filter-feeding organism stage' is essential.
Then, another giant leap. This was to cnidarians, the other marine
group considered to have very ancient (Precambrian) origins, and one
that includes corals, jellyfish, and sea anemones. The biological inno-
vations here were a definite gut, simple organs, and—importantly for
interaction with the physical environment—the beginnings of mus-
cle. Cnidarians also possess nerve tissue, and some modern cnidar-
ians have eyes. Crucially, some cnidarians also developed into
predators. In time, the act of eating other animals was to produce an
intense arms race in the oceans, but in the late Precambrian world this
had not yet happened, or at least there is little fossil evidence for it.
The fossil record of the Late Precambrian also does not preserve a
record of how animals evolved guts and other complex tissues,
although molecular clocks suggest that cnidarians were present 600
million years ago.
There are other branches of the early story of multicellular life.
Weirdest of all are the 'flat animals', the closest evolutionary affinities
of which science cannot agree on. Are they simple animals that are
closely related to sponges, or are they the descendants of more com-
plex animals that have lost much of their morphology? Whichever,
flat animals, more correctly called Placozoa, are small (millimetre-
scale), flat, and irregularly shaped. They resemble amoebae in size and
form, although they are multicellular. Imagine a world where animal
organization has progressed only to the level of Placozoa. It is a world
'peopled' by organisms quite reminiscent (if tiny versions) of the alien
invader of the 1958 comedy-horror movie The Blob , not least because