When viewing evolution over time, scientists generally favor parsimony, that is, lineages radiating from a common ancestor share most of the ancestor's features. This also means that animals at the bottom have simple features, which may grow more complex or which may become more specialized as other animals evolve from this common base. So, in ocean animals, sponges branch off from the common ancestor first; they are multicellular animals without much specialization. Jellyfish, sea anemones, and corals come later, each have multiple cell types; further, there are some cells in an outer layer surrounding the body and an inner tissue layer lining the gut. An animal with all these features plus nerve cells, a rudiamentary brain, and a middle tissue layer that forms muscles is thought to have given rise to comb jellies and the rest of animals.
So, transitions to new species create innovations, not only in body structures but in shared molecular processes. For example, many of the same proteins stick cells together and communicate messages between cells in all living animals. The same hold trues for muscles and CNS, which consist of multiple distinct parts built by networks of proteins encoded by genes [prior blogs have noted that this statement is a tad simplistic, but I am describing generalities for now].
Scientists expected animal genomes to mirror this pattern. It was thought that humans with 22,000 genes would mean that sponges, sea anemones, and comb jellies would have less. In 2007 this view was questioned when the starlet sea anemone was found to have nearly as many genes as humans [and of course, as noted in prior posts, genes are only part of the picture; there is so much more complexity that this type of number counting]. So the potential for complexity may have existed much earlier than thought.
Then the comb jellies upset the apple carts. An evolutionary tree built according to similarities in stretches of DNA, rather than shared anatomical traits, places comb jellies below the brainless sponges. Comb jellies are gelatinous like jellyfish, but they are so much more. In body plan they resemble sea anemones, corals, and other cnidarians [see http://en.wikipedia.org/wiki/Cnidaria], a group dating back 550 million years [and you were wondering when I would get to the Cambrian non-explosion]. Cnidarians have nerve cells that form a loose network in their bodies, but comb jellies have a seemingly more sophisticated nervous system with a rudimentary brain and synapses also found in flies, humans, and most other animals.
But, examination of the genomes of two species of comb jellies suggests they are primitive animals, and not part of the sea aneomes or corals, as originally believed. It is possible that comb jellies evolved before the brainless, gutless, muscle-less sea sponges. Though only a small fraction of animals, comb jellies threaten what was the accepted wisdow regarding the origin and evolution of animals.
So, this leads to a number of fascinating possibilities. Radical Thought #1, Comb jellies evolved nerves, muscles, and other complex features which sort of resemble that of humans, but independent of the ancestor that lead to most animals.
Radical Thought #2. The last common ancestor of all animals possessed complex features that remained in the comb jellies, but were lost without a trace in sponges, jellyfish etc. Multifaceted features are not suppose to be created and then vanish over evolutionary time.
One group of researchers decided to tackle the issue by assessing an entire comb jelly genome. They chose Mnemiopsis leidyi, present in quantity off Cape Cod and easily reared in the lab. Now here is where the tools of science show their rough edges. One mathematical model that compared the comb jelly genome with other organisms lead to the conclusion that comb jellies belong at the base of the tree of life, even with all their advanced features. How did this happen? Well comb jellies lack all sorts of components for encoding proteins; signaling pathways involved in cell growth and metabolism are also missing in comb jellies and sponges. Comb jellies lack microRNA and the molecular machinery to make them. One theory that then arises is that the mising genes evolved in other animals after comb jellies branched off from the ancestor of all other animals. Other researchers using different techniques came to a similar conclusion.
Yet, this seems absurd given that comb jellies have a brain (of sorts), a nervous system, and complex reactions. Some species of comb jellies chase prey, while others use their tentacles like fishing nets. There is another possibility then, one that has been mentioned in prior posts questioning if the Cambrian explosion was really an explosion, or merely an artifact of what survives as fossils, with the soft body creatures that existed before being rare in the fossil record and creating the illusion of an explosion once hard-bodied animals began to be present.
My personal favorite, I admit, reflects the reorientation of time and evolution. Just like it was once thought that DNA came first, then RNA, it now seems much more likely that RNA preceded DNA. So, back we travel to the Ediacaran Period (635 to 542 million years ago). See http://en.wikipedia.org/wiki/Ediacaran. This is the period before the Cambrian. Animal life in this period, before most modern lineages arose, was strange. Shapes and features were weird, given what we have come to believe is what life looked like as reflected in the Cambrian. Many Ediacaran creatures cannot be assessed using modern classifications.
Where does this take us? We know that life has cycled through time on Earth, with destruction on a global scale often destroying most life, and a few species surviving to differentiate and thrive. We humans owe our existence to at least one such global disaster. We forget that even after we came to be homo sapiens we came seriously close to extinction as a species "just" before expanding out of Africa. So, could comb jellies be the sole survivor of what existed during the Ediacaran? Given the development of the common lineage leading to animals as we generally conceive of them, it makes more sense than trying to twist and distort evolutionary theory and all the hard evidence in support of it to accomodate a single animal type, comb jellies.
In Darwin's day the contrarians said complexity could not evolve without a creator, an argument mirrored in many pseudo-scientific theories that are really reflections of one view of Christian faith. Now, scientists argue that complex systems may evolve, but not often. Why not? A billion years is a long time. Why is it not possible that complex systems have developed more than once, and the comb jellies are the survivor of an earlier experiment in the Ediacaran? No twists, no contortions. Science has always favored the least complex explanation, and this may be it.
Articles on this topic can be found at: http://www.nature.com/nature/journal/v452/n7188/full/nature06614.html; http://www.biomedcentral.com/1471-2164/13/714; http://www.evodevojournal.com/content/1/1/9; http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024152; http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0033261; http://www.biomedcentral.com/1741-7007/10/107; http://www.ncbi.nlm.nih.gov/pubmed/20029182; http://www.ncbi.nlm.nih.gov/pubmed/22116879.