Revolution #66, October 22, 2006
Tiktaalik and the Evolution of Land-roaming Animals
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A model of Tiktaalik roseae, depicted in what scientists believe to be the animal's environment about 375 million years ago. |
For more on Tiktaalik see: |
In April, the science journal Nature published news of an exciting new discovery. A group of paleontologists had found fossilized skeletons of a 375-million-year-old fish that is an evolutionary intermediate between fish and the first early amphibians. (Amphibians are cold-blooded vertebrates, animals with a spinal cord that typically live on land but breed in water, like frogs and salamanders.)
H. Richard Lane of the National Science Foundation said of the new finds, “These exciting new discoveries are providing fossil ‘Rosetta Stones’ for a deeper understanding of this evolutionary milestone—fish to land-roaming tetrapods.” (The Rosetta Stone, found in 1799 with Greek and Egyptian hieroglyphic inscriptions on it, was key to the deciphering of hieroglyphics. Tetrapods are vertebrate animals with four limbs or limblike appendages.)
Shallow Water Fish
The new species has been named Tiktaalik roseae. The word Tiktaalik (pronounced tic-TAH-lick) means “shallow water fish” in the language of the Nunavut people who live in the Canadian arctic, where the fossils were found.
Tiktaalik appears to have been a predator from 4 to 9 feet long, with sharp teeth and a head resembling a crocodile.
From analysis of fossil evidence, scientists have known for some time that the early amphibians that first fully took to land (and that later gave rise to reptiles, birds, and all mammals, including humans) evolved from lobe-finned fish. Lobe-finned fish have fins supported by a central appendage—made of bones and muscles—that is potentially useful for supporting the body on land, and some of them developed the ability to breathe air.
Scientists can establish relationships between fossils by comparing their physical features and ages (and also living species). They can group and separate species and lineages according to features they share in common or don’t share, even drawing out a graphical picture of this called a phylogenetic tree.
Before the discovery of Tiktaalik, paleontologists had already found a series of evolutionary intermediates from the middle to late Devonian geologic period (365-385 million years ago) linking the lobe-finned fish to early tetrapod amphibians. But the understanding of the steps in the major transformations of anatomy had remained quite limited. Tiktaalik appears to be a clear intermediate in body structure and age, shedding new light on the transition. Dating methods place it in between a fish with some tetrapod traits—Panderichthys, which lived about 385 million years ago—and the first clearly tetrapod species, Acanthostega and Icthyostega, of 365 million years ago.
The exact ancestral lines and relationships of these various species is not fully fleshed out, and more remains to be understood. But as a whole they provide proof of the evolution of amphibians from lobe-finned fish.
Tiktaalik retains many features of fish—a primitive jaw, scales, bone structure indicating it had gills to breathe, etc. The placement of its eyes on the top of its head shows that it spent much time under the water at the bottom looking up. But Tiktaalik has a mobile neck and a rib structure more characteristic of early amphibians.
Most striking are Tiktaalik’s fins. The co-leader of the project that found the fossil, University of Chicago Professor Neil Shubin, said, “Most of the major joints of the fin are functional in this fish. The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals.” Tiktaalik’s distal (farthest away from the wrist) fin bones resemble primitive digits, such as amphibians have.
Shubin believes the origin of land animals’ limbs probably involved further development and change of the features seen in the Tiktaalik fin.
Tiktaalik’s fin-limbs and skeleton indicate it “could support its body under the force of gravity whether in very shallow water or on land,” according to Farish Jenkins from Harvard University, co-author of the Nature article. Together, this evidence suggests Tiktaalik lived in shallow water but was also able to breathe air and may have even been able to move about briefly on land.
The Tiktaalik fossils were found on Ellesmere Island, 600 miles from the North Pole. This region contains exposed river sediments from the Devonian period, where fossils from the fish-amphibian transition were likely to be found. The scientists planning the search knew that during the Devonian, this part of North America had been part of a single land mass located at the equator, where it was known other species marking this transition had originally lived. That land mass has broken apart and shifted in the hundreds of millions of years since then to what exists now.
The Truth of Evolution
The Tiktaalik fossils are a snapshot of a transition which took place over tens of millions of years. It wasn’t “bound to happen” that fish evolved into tetrapods, but it did occur—through a whole process of genetic mutation, natural selection, and other evolutionary processes.
The transition from water to land is almost certainly an example of “adaptive radiation.” This is a well-known process in evolution whereby creatures that evolve certain novel traits (such as primitive limbs to walk or ways to breathe air, arising from genetic transformations and mutations that are inheritable) are able to move into new habitats with new survival advantages (due to new food sources, less danger of predators, less competition with other species, etc).
Being able to live in previously unoccupied habitats, first in shallow waters and then on land, would likely have given a survival advantage to the creatures undergoing this change, and therefore a “reproductive advantage,” allowing the newly inherited features to be spread through the population. Under these conditions, arising species can relatively quickly (in some cases in a time span of only thousands of years) bud off other new species in the bush of life.
The Tiktaalik finds have happened at a time when evolution and the scientific method itself are under attack from religious fundamentalists, from the President on down. And the Creationists, including “intelligent design” forces (who argue life is too complex to have evolved and must be the work of a supernatural designer, i.e., “God”), have attacked the significance of the Tiktaalik.
But this new discovery knocks even more holes in the claims of Creationists that the fossil record doesn’t show transitional links between such different forms of life. As Michael Novacek, a paleontologist at the American Museum of Natural History in New York City, said in the New York Times, “We’ve got the Archaeopteryx (transitional fossil linking reptiles to birds), an early whale that lived on land, and now this animal showing the transition from fish to tetrapod. What more do we need from the fossil record to show that the creationists are flatly wrong?”
To be clear, it is not the case that the Tiktaalik find is important because evolution needs more confirmation. All lines of scientific inquiry from every field of science for the past 150 years have firmly established that evolution happened, just as firmly as we know that the earth isn’t flat. The Tiktaalik finds provide more insight into how life evolved:
First, in an overall sense, scientists have found key evidence of transitional intermediate fossils linking lobe-finned fish to tetrapods, once again confirming life has evolved through a process of “descent with modification,” as opposed to notions of any intelligent design or creation.
Second, the finds further fill in the picture of specific form changes and timing in this profound transition of life from water to land. Tiktaalik features are snapshots in the evolution of fins to limbs, in skull shape, development of a skeleton that supports air breathing, etc. And scientific dating methods also place Tiktaalik fossils as intermediate between the more fish-like fossils and the clearly tetrapod fossils that have already been found.
Finally, Tiktaalik limbs and other structures reveal (as have many other discoveries) evolutionary pathways where ancestral features and structures with certain functions can become modified, through genetic mutation and the process of natural selection, to allow entirely new functions—opening up the possibility of speciation and adaptive radiation.
Learning about the truth of evolution is essential if people are to have a scientific and accurate understanding of the world and how it changes.
(We urge readers to delve deeper into the exciting world of evolutionary science. See Ardea Skybreak’s book The Science of Evolution and the Myth of Creationism, published by Insight Press. Go to www.insight-press.com for more information.)
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