November 11, 2010 Leave a comment
Timeline, 2008: One of the greatest quests in animal biology is the search for the “ur-animal,” the proto-creature that lies at the base of the animal family tree. For many years, the sponge held the low spot as most primitive animal, a relatively simple cousin of ours consisting of a few tissues and a tube that filters water for nutrients.
An animal, simple and alone
Research now suggests, however, that there’s an even older cousin at the base of the tree, an animal-like organism with three cell layers and four cell types that moves by undulating and exists alone in its own taxon. This organism, with the species name Trichoplax adhaerens, is the sole living representative of the Placozoa, or “flat animals.” It has been a bit of a mystery creature ever since the amoeba-like things were first noted a century ago in a German aquarium. Today, new techniques in systematics, the study of how living things are related, have helped pinpoint its place on the tree of life. These techniques have, however, left us with a few complex questions about this “simple” animal.
The animal kingdom family tree gets complex fast, with an early division in the trunk. Whatever lies at its base—and we’re still not sure what the organism was—that ancestor yielded two basic animal lineages. One led to sponges (Porifera) and branched to Cnidarians—corals, hydras, and organisms like jellies that have primitive nervous systems. The other lineage is the Bilateria, which includes everything from flatworms to bugs to beluga whales to us. Obviously, this lineage also has developed a nervous system, in many cases one that is quite complex.
Why a sponge will never be nervous
Earlier thinking was that the sponges, in lacking a nervous system, represented some earliest ancestor from which the two lineages sprang. But recent molecular analysis of the sponge and placozoan genomes has left a few systematists scratching their heads. The big headline was that the sea sponge was no longer the most primitive or oldest taxon. That honor may now belong to the placozoan, although some analyses place it as having evolved after Porifera.
An ur-animal? Or just another ur-cousin?
It may go a ways back, but it’s not so far that scientists can say that T. adharens is the “ur-animal,” or “mother of all animals.” In fact, it appears to share a few things in common with Bilateria, such as special protein junctions for holding tissues together, but computer analysis places it squarely in the Cnidaria branch of the animal family tree. Thus, this “weird, wee beastie,” while possibly older than the sponge, is still not the proto-ancestral animal condition. Rather than being the “ur-animal” from which all other animals sprang, it’s more like an “ur-cousin” of Bilateria.
Nervous system genes but no nervous system?
T. adhaerens also represents another conundrum for evolutionary biologists and systematists. It and the sponge both carry coding in their genomes for neural proteins, yet neither have nervous systems. The Cnidarian nervous system itself may have evolved parallel to that of the Bilateria, an evolutionary phenomenon known as “convergent evolution.” When evolutionary processes occur in parallel, unrelated species may share adaptations—such as a nervous system—because of similar selection pressures. The results are what we call “analogous structures,” which have similar functions and may even seem quite similar. But they are shared because of similar evolutionary pressures, not because of a common ancestry.
Lost traits make evolutionary biologists tear out their hair
The primitive placozoan does not have a nervous system, although organisms that arose later in the Cnidarian lineage, such as jellies, do. Yet those neural genes in its genome leave an open question and a continuing debate: Is the placozoan an example of another common evolutionary phenomenon in which a trait arises, but then is lost? Some scientists have suggested that there may have been an even older version of a nervous system, predating the Cnidarian/Bilateria split. This trait then vanished, leaving behind only these traces in the primitive placozoan and sponge genomes. With this scenario, the two nervous systems would have a shared ancestry: instead of being analogous traits resulting from convergent evolution, they would represent homologous traits, shared because of a common neural ancestry.