Nov 09 2017

Evolution Caught in the Act

The hypothesis that life on Earth as it is currently found is the result of biological evolution from a common ancestor over billions of years is supported by such a mountain of evidence that it can be treated as an established scientific fact. Further, it is now a fundamental organizing theory of biology.

This, of course, does not stop ideologically motivated denial. There are those who have been systematically misinformed about the evidence, and the nature of science itself. What they think they know about evolutionary theory they learned from secondary hostile sources. One of the common lies they are repeatedly told is that there are no transitional fossils.

This claim amazes me still, because the evidence is so easily accessible. Lists of transitional fossils are easy to find. One of my favorite examples is the evolution of birds, because the morphological transition from theropod dinosaurs to modern birds was so dramatic.

I also have to point out that this evidence represents a successful prediction of evolutionary theory. When Darwin first published his theory the fossil record was scant. Enough fossils had been discovered for scientists to see that life was dramatically changing over geological time, but the puzzle was mostly empty. There were not enough specimens to see connections between major groups. Evolutionary theory predicts that such connections would be found – and they were, and they continue to be.

The fossil record is such a slam-dunk win for evolutionary theory that deniers have no choice but to simply lie and falsely state that they don’t exist. They try to divert attention to the remaining gaps in the record, or the occasional fossil hoaxes. When you point out the many dramatic transitional fossils they perform the intellectual equivalent of sticking their fingers in their ears and saying, “La, la, la.”

The most dramatic transitional fossils relate to evolutionary changes resulting from a major change in lifestyle. When dinosaurs took to the wing, for example. Or whenever creatures adapt from the sea to land, or from the land back to the sea. Whales are a great example. We now have a compelling sequence of transitional whales, and can see the slow loss of legs over time, the movement of the nostril to the top of the head, the increase in size, and the development of flippers. Ambulocetus is about half way through this transition – a literal walking whale.

In addition to the well-known groups, there are many nicely documented transitions in less well-known groups – for example, the pleurosaurs. These are ancient reptiles that went back to the sea and evolved to an aquatic lifestyle. They are similar in this way to the plesiosaurs, ichthyosaurs and mosasaurs.  A recent specimen was discovered that is 155 million years ago, is remarkably well-preserved, and represents a clear transitional species. As Science reports:

The creature (which the scientists dubbed Vadasaurus, Latin for “wading lizard”) lived 155 million years ago and didn’t have the elongated trunk or relatively shorter limbs that later aquatic species of pleurosaurs did, the researchers report today in Royal Society Open Science. So, Vadasaurus would have been less streamlined overall than its aquatic kin, they suggest. But other features, such as the shape of its skull and the shape and placement of its nostrils, hint that some aspects of the creature were indeed becoming more adapted to an aquatic lifestyle.

So it was partly adapted to the sea, but not completely. Later specimens show more complete adaptation to the water. The specimen also had less ossification, meaning lighter bones, than its terrestrial ancestors. Lighter bones would be an aquatic adaptation – they would make floating easier and heavier bones would not be necessary for support in the water.

Aquatic adaptation is an excellent window into evolutionary change, because life in the water produces a suite of strong selective pressures. You can survive in the water, for example, with stumpy legs, but they are just getting in the way. They slow you down, so there is continuous selective pressure for smaller legs. Therefore we see in the fossil record progressively smaller hind limbs in groups adapting to the sea. Modern whales are left with just an internal bony vestige.

Another strategy evolution deniers use to sow doubt and confusion about the fossil evidence is to focus on tiny details and ignore the bigger picture. Here is a good example from the Orwellian named,  Evolution News. They report on two new transitional fossils, including another feathered dinosaur. The author does not acknowledge that such specimens fill in the morphological space of already known species, and therefore are transitional, providing further evidence for evolution. Rather, they argue, that because these specimens change the way we draw the lines of descent they are evidence against evolution.

That is a common tactic- misinterpret disagreement or uncertainty about the details as if it calls into question the bigger reality. Scientists are trying to piece together exactly what evolved from what when based upon an incomplete record. This is like trying to put jigsaw puzzle pieces in the right place when you only have 10% of the pieces. Every time you find a new piece there is the chance that it will change where you think the pieces go.

If evolution were not true, however, we would not be finding any pieces, or we would be finding pieces to other puzzles entirely. Once we started digging up fossils we could have found a complete absence of life prior to 10 thousand years ago. We could have found that species are stable throughout geological history. We could have found different species, but ones with not possible relationship to extant species.

That is not what we found. We found, as evolutionary theory regarding common descent predicts and requires, dramatically and sequentially changing multicellular species going back 550 million years. Further, fossil species largely fit into a compelling evolutionary pattern. We find creatures that are plausible ancestors to living creatures. We don’t find fossils that are impossible chimeras or totally out of sequence.

However, when you drill down to the details, the fossil record does not always provide enough evidence to make precise reconstructions. Scientists interpolate as best they can from existing evidence, but during this phase of discovery new evidence can significantly change how these maps are drawn. That does not call into question the fact of common descent itself. Pretending it does is intellectually dishonest, which is the hallmark of evolution deniers.

The fact remains, with each new transitional fossil discovered, there is another vindication for evolutionary theory.

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