Transitional forms: muscle proteins

How old is evolution? How conservative? Try this: the proteins that an amoeba (ameba) uses to move are essentially the same as those that make up the muscle fibres in a vertebrate such as ourselves.

Though it appears to have nothing in common with muscular movement, ameboid movement probably depends on contractile components and mechanisms surprisingly similar to those in the muscle cells of animals. …[T]he cytoplasm of an ameba is found to contain thick and thin microfilaments similar in appearance and dimensions to thick (myosin) and thin (actin) microfilaments of striated muscle. Extracts of muscle myosin appear to cross-react with the thin filaments from amebas. And cytoplasm from amebas uses the nucleotide adenosine triposphate (ATP) as an energy source for movement, as muscle does.

From Living Invertebrates, page 27 (chapter on “Protozoans”), by Vicki Pearse, John Pearse, Mildred Buschbaum, & Ralph Buschbaum.

A. sediba is a splendid transitional fossil

A man holding a small, semi-human fossil skull

Lee Berger with A. sediba

Look at the quiet, yet awed, delight on this palaeontologist’s face. That is the reward of science. He has been analyzing two almost complete skeletons of Australopithecus sediba found together in South Africa.

Another gap in human evolution has been decisively filled. Australopithecus africanus, discovered by Raymond Dart in the 1930s, was a climbing, ape-like hominid. Homo habilis and Homo erectus were human-like and apparently tool–using. A. sediba seems to be right between them. It has longer legs than A. africanus and walks bipedally. It has features found in both A. africanus and H. habilis. Its hand is more like our own than is that of H. habilis. Carl Zimmer explains: The verge of human. Between this fossil and Ardipithecus ramidus, we seem to be getting a grasp on ape evolution.

Transitional fossils: halfway flat fish

National Geographic’s 2009 roundup of transitional fossils included a fish that is evolving to lie concealed on the ocean floor. Modern flatfish are born symmetrical, with eyes an other side of their head, but as they age one eye moves around to the other side so that the fish can watch for danger with both eyes while lying flat on the sea floor.  The lack of “halfway” forms has been used to argue against evolution. Now, one has been found. This one appears to have one eye just over the midline. I wait for anti-evolutionists to acknowledge that the theory of evolution has been confirmed once again: Amphistium, transitional fossil, a half-way flat fish.

fossil fish

Amphistium, a transitional fossil in flatfish evolution

Transitional seal fossil

A major transitional fossil has been found. It’s a seal ancestor, 23 million years old. It a walking seal that looks a bit like an otter.

Puijila darwini

Puijila darwini

This is the oldest fossil of an ancestor to seals that has been found. Pujilla means “young sea mammal” in the Inuktitut language as spoken near where the fossil was found, on Devon Island. The species name darwini honours Charles Darwin. The great biologist hypothesized that land mammals would be likely to adapt to fresh water first and only later take to the sea. Pujilla, found in sediments laid down in a fresh water environment, is evidence for that hypothesis.

“The find suggests that pinnipeds went through a fresh water phase in their evolution,” said Natalia Rybczynski from the Canadian Museum of Nature (CMN) in Ottawa, who led the fieldwork.Pinnipeds are the seals, sea lions, and walruses. (Manatees returned to the water in a separate adaptation.)

“It also provides us with a glimpse of what pinnipeds looked like before they had flippers.”

The skeleton was about 65% complete, which enabled the researchers to reconstruct what the animal would have looked like in remarkable detail.

The legs suggest it would have walked upright on land; but the foot bones hint strongly at webbed feet.

The fossil was found in a former crater lake. Scientists have also found fossil fish from the same period, which may have been prey for the semi-aquatic seal.

“The remarkably preserved skeleton of Puijila had heavy limbs, indicative of well developed muscles, and flattened phalanges (finger or toe bones) which suggest that the feet were webbed – but not flippers,” said Mary Dawson from the Carnegie Museum of Natural History in Pittsburgh, US, another of the scientists.

The teeth indicate that the animal was a carnivore.

Until now, the most primitive fossil pinniped was a creature called Enaliarctos that dates from about the same period and appears to have lived in the sea along the northwestern coasts of North America.

Enaliarctos had flippers, but may have had to bring its prey to the shore for eating, whereas modern pinnipeds manage it at sea.

Intriguingly, different species of present-day seal swim in different ways – either rotating their flippers, or waving their hind-quarters from side to side, using the hind limbs for propulsion.

Enaliarctos appears to have been capable of both modes of swimming – and as a four-legged animal with four webbed feet, Puijila is a logical fore-runner of this creature which could swim with all four limbs.

This discovery suggests that seals, sea lions, and walruses evolved in the Arctic, and northern Canada and northern Russia likely places to look for more ancestral seals.

Brian Switek at Laelaps has detailed information from the actual paper in Nature.

Transitional forms: Tiktaalik and others

Here’s a brief review of several lovely transitional fossils in the tetrapod line, which tie together lobe-finned fishes with early land amphibians. The latest discovery is, from 300 million years ago, Tiktaalik roseae:Hello, Beautiful.”

a famous series of transitional fossils in the tetrapod lineage

Tiktaalik is an example of the predictive power of evolutionary theory. The discoverers noted the kind of fossil that they were missing, its expected environment, and the approximate age that it should be, then looked in rocks of the right age and geological origin in northern Canada. And voila! There was their fossil.

Yanoconodon, a new transitional fossil

A big tip and flourish to PZ Myers at Pharyngula for this detailed summary of an article from this week’s Nature. Yanoconodon, a primitive mammal, fills yet one more gap in the evolution of the inner ear bones from jaw bones:

“The latest Nature reveals a new primitive mammal fossil collected in the Mesozoic strata of the Yan mountains of China. It’s small and unprepossessing, but it has at least two noteworthy novelties, and first among them is that it represents another step in the transition from the reptilian to the mammalian jaw and ear.”

Tiktaalik is transitional between lobe-finned fishes and tetrapods

News is flashing around the world about a spectacular fossil. The Ottawa Citizen has a nice article about it: the finding of a large fish with bony fins, sturdy ribs and a bendable neck, but fish scales. It fulfills the predictions of evolutionary scientists about the anatomy of a transitional fossil between lobe-finned fishes and primitive tetrapods. It fits in neatly between Panderichthys and Acanthostega.

Scientists searched for four years at the right level (375 million years ago) in the right environment (former shallow swamps) before finding three well-preserved, three-dimensional fossils of vaguely crocodilian fish four to nine feet long.

Read a layman’s article at The Harvard Gazette: “Missing Link Crawls out of Muck” or the scientific viewpoint at Living the Scientific Life: “Missing Link” and Pharyngula: “Tiktaalik makes another gap”.

Transitional species in insects: cockroaches to termites

“MrDarwin” at Darwinblog has posted a lovely example of insect evolution recently figured out:

If termites have indeed evolved from cockroaches, then we should expect to find transitional living or fossil termite-like cockroaches and cockroach-like termites. In addition, other lines of evidence should be consistent with this relationships. For example, the derived taxa (termites) should not predate their ancestors (cockroaches) in the fossil record. Moreover, the earliest members of the derived group—that is, the ones closest to the evolutionary divergence—should be the most similar to the ancestral group. Finally, the phylogenetic relationships of their symbiotic bacteria and protists, which are utterly dependent upon their hosts for survival, should be similar to the phylogenetic relationships of their hosts.

So far, all of those things have been found, or not found, according to the predictions of evolutionary theory.

Follow this link for more about termites.

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