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Earth’s first animal was the ocean-drifting comb jelly, not the simple sponge, according to a new find that has shocked scientists who didn’t imagine the earliest critter could be so complex.
What’s wrong with this? Basically, the study showed that the line leading to the ctenophores may have diverged before the other metazoan lineages. Stating comb jellies were the “first animal” is going to lead to people thinking that all animals evolved from comb jellies! Some other stories on this article do specify the ctenophore lineage diverged first, which is somewhat less misleading. However, this lineage diverging first does not mean modern ctenophores were there at the time. We might also consider that at that divergence the other lineage produced led eventually to humans, and we certainly were not swimming about in the ocean with the ctenophores waiting for land-living plants and animals to evolve so we could get out and dry off. Modern humans are much different from their early ancestors, and modern ctenophores may be as well.
EVOLUTIONARY TREES will show up fairly frequently here, and are also frequently misunderstood, so I will present a summary of Gregory’s excellent article “Understanding Evolutionary Trees“. I strongly suggest reading it (free full text!) because while I have shown a few of his cladograms he has many more in the paper and goes into much more detail regarding common misunderstandings about evolutionary trees. Some of these misunderstandings are related to the way we tend to think about evolution, some are due to unfamiliarity, and others are learned from obsolete presentations of the process of evolution. Even people who are familiar with evolution may need to pause once in a while and think “Is that a correct interpretation?” when looking at evolutionary trees.
IN THE PAST few years there has been some acrimony regarding the division of the protostomes into Lophotrochozoa and Ecdysozoa. This decision split the annelids and the arthropods, and placed nematodes in with the arthropods. At its first proposal support for this tree was weak, but successive discoveries have strengthened it. Now the most detailed tree yet confirms this division, and clarifies other relationships.
I RECENTLY READ a paleontology book that irked me by referring to the basal synapsids as “mammal-like reptiles”. This term pre-dates modern phylogenetics. It violates several rules of phylogenetics, the first being that all clades (groups of related organisms) should contain organisms sharing a single common ancestor in that clade, and the second being that clades should nest within each other, so that each more recent clade fits into its ancestors’ clades as well. In other words, clades should not be polyphyletic, but monophyletic. An example of a polyphyletic group is the pachydermata, an obsolete category containing elephants, hippopotami, rhinoceri, horses, tapirs, and pigs simply on the basis that these organisms have thick skin and hooves. Now these animals are placed into separate clades since they do not share a recent common ancestor. Polyphyletic groups are a great embarrassment to phylogenicists.