This week there's been two good articles about how much of the machinery of animals dates far back. The first is by Frank Zimmer in The New York Times, titled "Plain, Simple, Primitive? Not the Jellyfish". The jellyfish is part of the cnidarians, the family relationship of which is depicted in the figure on the left (click on image to view it in full scale). Cnidarians are radially symmetrical, meaning that they're symmetrical around several axes, like the spokes in a bicycle wheel, whereas all the organisms depicted to the right of the cnidarian are bilaterally symmetrical, meaning they're only symmetrical around the head-to-tail axis (except for the echinoderms, which evolved radial symmetry independently). Bilateral animals use a special genetic toolkit for constructing their bodies, and cnidarians were thought to be an evolutionary relic from before this toolkit evolved. But recent findings have overturned this belief:
"Much to their surprise, the scientists found that some genes switched on in embryos were nearly identical to the genes that determined the head-to-tail axis of bilaterians, including humans. More surprisingly, the genes switched on in the same head-to-tail pattern as in bilaterians.
Further studies showed that cnidarians used other genes from the bilaterian tool kit. The same genes that patterned the front and back of the bilaterian embryo, for example, were produced on opposite sides of the anemone embryo.
The findings have these scientists wondering why cnidarians use such a complex set of body-building genes when their bodies end up looking so simple. They have concluded that cnidarians may be more complicated than they appear, particularly in their nervous systems. "
The second article is from this week's Nature, "Back to our roots" (registration required) by Helen Pilcher. It shows how the genetic toolkit has been pushed even further back in time, to before the evolution of sponges, indicating they were present in the urmetazoan, the common ancestor of all animals:
"There are signs that many other molecules associated with development in animals also occur in sponges. The Wnt family of proteins, for example, influences how cells become specialized and also helps to lay down the key spatial coordinates of the body plan in complex animals. Sponge cells make the Frizzled protein, a receptor that is activated by Wnt proteins. And they also make a variety of metazoan-like transcription factors – proteins involved in controlling gene expression – that are key players in development.
The fact that these genes occur during development in all existing animal lineages hints that they were playing a regulatory role in the embryos of the first metazoan. "The urmetazoan was probably quite sophisticated in a developmental and genomic sense," says [Bernard] Degnan [who is a geneticist at the University of Queensland]. This suggests that it already had the genetic toolkit to direct a body plan containing multiple cell types.
To find out where this toolkit came from, biologists are looking even further back in time, at the single-celled ancestors of the urmetazoan. Their modern-day descendants are choanoflagellates, unicellular creatures that look uncannily like sponge collar cells. Surprisingly, choanoflagellates harbour many of the tools needed for multicellular living."
This sheds light on a possibility off-handedly proposed by Michael Behe and later developed by Mike Gene: What if the first lifeforms were designed, containing the structures needed for the evolution of more complex organisms? If this is the case, I'd expect this "genetic toolkit" to trace back to unicellular organisms.
Image Â© Nature, 2005.