One consistent misapprehension a lot of people have about evolution is that the process of natural selection always leads toward organisms becoming stronger, smarter, faster, and more complex.
As my evolutionary biology professor put it, this is incorrect because at its core, "evolution is the law of whatever works." The most successful, widespread, diverse, and numerous animals on Earth are, by far, insects -- they're not necessarily any of the aforementioned things (especially smart), they are just exceedingly good at reproducing fast and filling available niches. Whatever traits happen to be selected for by the environment at the time result in the direction evolution takes. And this can change if the environment changes -- as has been observed in a number of fossil lineages where the average body size increased for a while, then reversed course and decreased.
Evolution is not goal-oriented. The idea that it's heading in a particular pre-determined direction is a holdover from the old Aristotelian idea of the scala naturae, where there's a ladder of increasing complexity and intelligence, with humans, of course, occupying the top rung. (At least until the concept was adopted by medieval Christian scholars; at that point humans got knocked down a couple of pegs, with the higher rungs taken up by angels and, at the top, God.) But you still hear people -- even scientific, rational types -- talk about "primitive" and "advanced" species, and ones being "highly-evolved" (or not), when the truth is that all modern life forms, from bacteria to birch trees to baboons, have exactly the same length of evolutionary history, going back to LUCA (the "last universal common ancestor") something like four billion years ago.
It's just that in those four billion years, some of them have changed a great deal more than others have.
Given that even people who are quite knowledgeable often still have that bias floating around, it can come as a significant shock to find out that there are some anatomically simple animals that are actually quite recently evolved -- and close to other species we consider "advanced." Two of the most striking examples are echinoderms (such as starfish and sea urchins, which undergo a peculiar decentralization during development, losing most of their sophisticated organs up to and including the central nervous system) and tunicates (sometimes referred to as "sea squirts," which superficially look like filter-feeding sponges but are actually some of the closest invertebrate relatives to vertebrates). In both cases, the larvae give away their actual placement in the family tree of life, as does their DNA; both of these groups represent fairly recent developments, as these things go.
Another example, and the reason this topic comes up, is Class Agnatha, which includes lampreys and hagfish, and sometimes are called "jawless fish." (The term "fish" actually has no evolutionary relevance; it lumps together very distantly-related groups, excluding others that are far closer cousins. Lungfish and coelacanths, for example, are more closely related to amphibians -- and thus to us -- than they are to your standard-issue fish.)
In any case, lampreys and hagfish are distinguished on the gross anatomical level by lacking lower jaws, and -- by the typical way of thinking about this -- must be some kind of "primitive" holdover from before paired jaws were developed by the rest of us vertebrates. It's true they branched off early, and are only distantly related to other vertebrates, but some research that came out last week in Nature Ecology & Evolution suggests that their lineage lost their lower jaws, not that our direct ancestors somehow gained them along the way.
The research looked at the genetic control over jaw development, and found that the pattern was strikingly similar between vertebrates with jaws and those without -- but that those without had switched off a gene called pou5 that guides cells in the neural crest, a cluster of cells in the head of the embryo that specialize to produce a number of different structures. Lampreys and hagfish have the gene, they just don't express it in the embryonic tissue that in other vertebrates leads to the mandible -- suggesting strongly that they evolved from ancestors that had it and expressed it.
"While most of the genes controlling pluripotency are expressed in the lamprey neural crest, the expression of one of these key genes -- pou5 -- was lost from these cells," said Joshua York of Northwestern University, lead author of the paper. "Amazingly, even though pou5 isn't expressed in a lamprey's neural crest, it could promote neural crest formation when we expressed it in frogs, suggesting this gene is part of an ancient pluripotency network that was present in our earliest vertebrate ancestors."****************************************
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