Sean Carroll has a great book, fun to read on this topic. highly recommend it:
Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom. Sean B. Carroll.
it's about all those homeobox genetic networks that guide metazoan development, and the surprise is that ALL metazoan phylla have similar networks, so it all evolved before all the earliest fossils we have for these boogers.
A) we are actually learning in the past 20 years how
cells repeatedly transform themselves into two non
identical daughter cells, and then further communicate
with each other to increase the differentiation
between themselves and eventually build a creature!
B) surprise: only 25,000 genes in large verebrates.
seems too simple to create a whole critter, especially
complex mammals. wait... we've found that many genes
involved in development have a whole slew of switches
behind them that can get turned on and off depending
where in the developing embryo these genes' cells are.
there's the complexity we are looking for. we missed
the existence of the switches with the old style
C) surprise: fish birds bats slugs bugs starfish worms
jellyfish are built with the SAME kinds of networks of
developmental genes and switches. for instance the
gene that turns on when eyes develop, are involved in
humans, octopi and jellyfish, even though these three
have vastly different eyes and we thought that they
came up with those eyes independantly.
D) this means that the creative bit that happened to
make metazoan bodies happened in some unknown common
ancestor to ALL metazoans before the cambrian
explosion. the plot thickens! the next common
ancestor going back that we all have is single celled
critters called Choanoflagellates. do THEY have these
genes and switches and what do THEY do with them?
E) the nature of the switches is such that we may have
discovered interesting mechanisms for evolution of
different body plans. it may be EASY with the flick
of one switch to turn a crab into a lobster or a
millipede into a grasshopper. (sort of) the current
dogma is that evolution happens gradually. for some
reason biologists have been resisting the possibility
of evolution by jumps (smells of creationism...) but
we are gathering evidence that evolution might also
occur by some high acceleration moves!
F) the genes have cool names like sonic hedgehog and
here's another review:
The New Just So Stories: PAULA M. MABEE
so in WHAT KIND OF CRITTER did this stuff evolve? is there some of it in the protistan ancestors to metazoans? i'd love to do a survey of the protistan phyla and see what's known about their behavior/cell physiology what would encourage/enable these critters to coordinate and become metazoan? for instance i found a paper that was easy to find on the web about similar genes in single celled choanoflagelates. there must be tons more.
i love singled celled critters. i should get my microscope out of storage and watch 'em again. i use to love watching stentor.
Science 18 July 2003:
Vol. 301. no. 5631, pp. 361 - 363
Evolution of Key Cell Signaling and Adhesion Protein Families Predates Animal Origins
Nicole King, Christopher T. Hittinger, Sean B. Carroll*
The evolution of animals from a unicellular ancestor involved many innovations. Choanoflagellates, unicellular and colonial protozoa closely related to Metazoa, provide a potential window into early animal evolution. We have found that choanoflagellates express representatives of a surprising number of cell signaling and adhesion protein families that have not previously been isolated from nonmetazoans, including cadherins, C-type lectins, several tyrosine kinases, and tyrosine kinase signaling pathway components. Choanoflagellates have a complex and dynamic tyrosine phosphoprotein profile, and cell proliferation is selectively affected by tyrosine kinase inhibitors. The expression in choanoflagellates of proteins involved in cell interactions in Metazoa demonstrates that these proteins evolved before the origin of animals and were later co-opted for development.
related to this is a discussion i was having about evolution of eye development some time ago:
my posts are blackskimmer