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A PAPER PUBLISHED recently in Nature details the discovery of a common ancestor of salamanders and frogs, Gerobatrachus hottoni, by Anderson and coworkers. This creature had a salamander-like build, but has a broadened skull like frogs. A variety of traits were studied to determine this organism’s relationships, such as the teeth, various skull bones, presacral spine, and otic notch. Its position in the early Permian places the frog/salamander divergence in the Middle Permian, about 270-260 million years ago. Of course this find is interesting, but I was perhaps more interested by the phylogenetic tree that they composed including this new species.
I HAD PLANNED on covering the article on the platypus genome that came out in Nature last week, but since then this paper has been discussed in detail on Pharyngula and Adaptive Complexity and I think further discussion would be moot. I did notice while reading the paper that the unfortunate description of certain platypus genes as “reptilian” cropped up frequently. Although the authors noted that the sauropsids and synapsids are amniotes, they never mentioned that platypus genes shared with reptiles are actually basal amniote genes. Although their phylogenetic tree shows synapsids and sauropsids clearly diverging from a common amniote ancestor, they do not seem to realize referring to these ancestral amniote genes as “reptilian” suggests evolution of the platypus (and thus all synapsids) from reptiles instead of from a non-reptilian amniote.
However, today I want to talk about the platypus’ sex chromosomes. Platypuses, like the therians, have genetic sex determination. They have an XX/XY system in which males (XY) are the heterogametic sex. Many reptiles have environmental sex determination, with sex determined by factors such as incubation temperature during embryonic development. However, some reptiles and all birds have a ZW/ZZ sex determination system, with females (ZW) as the heterogametic sex.
LET’S SUPPOSE that we are interested in studying the evolution of the squamates, the snakes and lizards. We know that if we track down the evolutionary tree of the squamates we find that they are lepidosauromorphs, and a sister group to the archosauromorphs. Ah-hah! The archosauromorphs diverged before the evolution of the lizards, so living archosauromorphs must preserve ancestral traits, we think. So we go forth and find an archosauromorph and sequence its genome. There are multiple species from which to choose, but we grab the closest one—the European starling. Since this is an archosauromorph and early diverging compared to the snakes and lizards that must mean that it is “primitive”.
Does this seem odd to you?
WHILE MANY NICHES are ever-present in vertebrate-dominated ecosystems, sometimes we find an extinct group that was unique in some way. At various times in the past carnivores have evolved saber-teeth, but currently there are no saber-toothed predators (although the clouded leopard may be working on it). In a previous post I mentioned Simocyon, a cursorial generalized carnivore that retained arboriality in order to escape from larger predators, including the saber-toothed cats. Today I will write about Thylacoleo carnifex, the so-called marsupial lion.
VOLVOCINE ALGAE have a recent evolution of multicellularity, only 30-70 million years ago. This may produce a better record of the early history of this process than we have for other multicellular organisms. Metazoans and multicellular plants evolved over 550 million years ago (multicellular plants multiple times, and some suggest a very ancient history of multicellular algae over 800 million years ago). The fossil record for fungi is not very good, but unambiguous multicellular fungi were present by 500 million years ago. Bacteria meanwhile beat everyone out by evolving multicellularity several times perhaps 2-3 billion years ago. Since most multicellular organisms have a distant origin, extinction has eroded the base of their evolutionary trees so that the details of the transition are hard to extract. The volvocine algae have a much more recent history of multicellularity, and we have been able to determine much about their evolutionary history from phylogenetic studies of these algae and their relatives.
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.
THE ATTINE ANTS are a clade of New World ants that cultivate fungal gardens for use as a food source. The most widely known group are the leaf-cutter ants, which spend most of their time harvesting slices of leaves that are chewed up and placed in the fungal gardens in their underground colonies. There are many other lesser-known species, and these practice different forms of fungal agriculture. A new study of the phylogenetics of attine ants show that the different methods of fungal cultivation emerged in single events.
I HAVE BEEN planning on writing this post for quite a while and hope to do the topic justice. The Cambrian explosion refers to the sudden radiation of metazoan life during the Cambrian period, and is usually placed between about 540-530 million years ago. It is often said that during the Cambrian explosion all of the modern phyla appeared. This statement has been seized upon by creationists who claim that the Cambrian explosion proves a sudden supernatural creation of life, rather than evolution of organisms over time.
THERE ARE certain niches that seem to be filled by one species or another at any time period. In my recent post on mammalian evolution, for instance, I mentioned Castorocauda, a Jurassic mammaliform that seems to have fit into the niche now occupied by beavers or otters. But occasionally we run across an animal that seems to be adapted for a unique role in its ecosystem. One of these animals is Simocyon. This is a puma-sized caniform that lived about 14 million years ago, and died out by four million years ago. Simocyon has a variety of unusual adaptations.
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.