Getting a Leg Up on Land [Evolution]

Scientific American ^ | December 2005 issue | Jennifer A. Clack

[PatrickHenry]

In the almost four billion years since life on earth oozed into existence, evolution has generated some marvelous metamorphoses. One of the most spectacular is surely that which produced terrestrial creatures bearing limbs, fingers and toes from water-bound fish with fins. Today this group, the tetrapods, encompasses everything from birds and their dinosaur ancestors to lizards, snakes, turtles, frogs and mammals, including us. Some of these animals have modified or lost their limbs, but their common ancestor had them -- two in front and two in back, where fins once flicked instead.

The replacement of fins with limbs was a crucial step in this transformation, but it was by no means the only one. As tetrapods ventured onto shore, they encountered challenges that no vertebrate had ever faced before -- it was not just a matter of developing legs and walking away. Land is a radically different medium from water, and to conquer it, tetrapods had to evolve novel ways to breathe, hear, and contend with gravity -- the list goes on. Once this extreme makeover reached completion, however, the land was theirs to exploit.

Until about 15 years ago, paleontologists understood very little about the sequence of events that made up the transition from fish to tetrapod. We knew that tetrapods had evolved from fish with fleshy fins akin to today's lungfish and coelacanth, a relation first proposed by American paleontologist Edward D. Cope in the late 19th century. But the details of this seminal shift remained hidden from view. Furthermore, estimates of when this event transpired varied wildly, ranging from 400 million to 350 million years ago, during the Devonian period. The problem was that the pertinent fossil record was sparse, consisting of essentially a single fish of this type, Eusthenopteron, and a single Devonian tetrapod, Ichthyostega, which was too advanced to elucidate tetrapod roots.

With such scant clues to work from, scientists could only speculate about the nature of the transition. Perhaps the best known of the scenarios produced by this guesswork was that championed by famed vertebrate paleontologist Alfred Sherwood Romer of Harvard University, who proposed in the 1950s that fish like Eusthenopteron, stranded under arid conditions, used their muscular appendages to drag themselves to a new body of water. Over time, so the idea went, those fish able to cover more ground--and thus reach ever more distant water sources--were selected for, eventually leading to the origin of true limbs. In other words, fish came out of the water before they evolved legs.

Since then, however, many more fossils documenting this transformation have come to light. These discoveries have expanded almost exponentially our understanding of this critical chapter in the history of life on earth--and turned old notions about early tetrapod evolution, diversity, biogeography and paleoecology on their heads.

Finding a Foothold

Among the first fossil finds to pave the way for our modern conception of tetrapod origins were those of a creature called Acanthostega, which lived about 360 million years ago in what is now eastern Greenland. It was first identified in 1952 by Erik Jarvik of the Swedish Museum of Natural History in Stockholm on the basis of two partial skull roofs. But not until 1987 did my colleagues and I finally find specimens revealing the postcranial skeleton of Acanthostega.

Although in many ways this animal proved to be exactly the kind of anatomical intermediary between fish and full-blown tetrapods that experts might have imagined, it told a different story from the one predicted. Here was a creature that had legs and feet but that was otherwise ill equipped for a terrestrial existence. Acanthostega's limbs lacked proper ankles to support the animal's weight on land, looking more like paddles for swimming. And although it had lungs, its ribs were too short to prevent the collapse of the chest cavity once out of water. In fact, many of Acanthostega's features were undeniably fishlike. The bones of the forearm displayed proportions reminiscent of the pectoral fin of Eusthenopteron. And the rear of the skeleton showed a deep, oar-shaped tail sporting long, bony rays that would have provided the scaffolding for a fin. Moreover, the beast still had gills in addition to lungs.

The piscine resemblance suggested that the limbs of Acanthostega were not only adapted for use in water but that this was the ancestral tetrapod condition. In other words, this animal, though clearly a tetrapod, was primarily an aquatic creature whose immediate forerunners were essentially fish that had never left the water. The discovery forced scholars to rethink the sequence in which key changes to the skeleton took place. Rather than portraying a creature like Eusthenopteron crawling onto land and then gaining legs and feet, as Romer postulated, the new fossils indicated that tetrapods evolved these features while they were still aquatic and only later co-opted them for walking. This, in turn, meant that researchers needed to reconsider the ecological circumstances under which limbs developed, because Acanthostega indicated that terrestrial demands may not have been the driving force in early tetrapod evolution.

Acanthostega took pride of place as the missing link between terrestrial vertebrates and their aquatic forebears. There was, however, one characteristic of Acanthostega that called to mind neither tetrapod nor fish. Each of its limbs terminated in a foot bearing eight well-formed digits, rather than the familiar five. This was quite curious, because before this discovery anatomists believed that in the transition from fish to tetrapod, the five-digit foot derived directly from the bones constituting the fin of Eusthenopteron or a similar creature. Ordinarily, scientists might have dismissed this as an aberrant specimen. But a mysterious partial skeleton of Tulerpeton, a previously known early tetrapod from Russia, had a six-digit foot. And specimens of Ichthyostega also found on our expedition to eastern Greenland revealed that it, too, had a foot with more than five digits.

PRIMEVAL PROMENADE: Ichthyostega is the earliest known tetrapod to show adaptations for nonswimming locomotion, although it seems likely to have moved more like a seal than a typical land vertebrate. This animal also had some aquatic features, including a large tail and flipperlike hind limbs, as well as an ear that appears to have been specialized for underwater use. How Ichthyostega divided its time between the terrestrial and aquatic realms is uncertain. But it may have dug nests for its eggs on land and hunted and fed in the water.

Findings from developmental biology have helped unravel some of this mystery. We now know that several genes, including the Hox series and Sonic Hedgehog, control elements of fin and limb development. The same sets of these genes occur in both fish and tetrapods, but they do different jobs in each. Hoxd 11 and Hoxd 13, for instance, appear to play a more pronounced role in tetrapods, where their domains in the limb bud are enlarged and skewed relative to those in the fish fin bud. It is in these regions that the digits form. How the five-digit foot evolved from the eight-digit one of Acanthostega remains to be determined, but we do have a plausible explanation for why the five-digit foot became the default tetrapod pattern: it may have helped make ankle joints that are both stable enough to bear weight and flexible enough to allow the walking gait that tetrapods eventually invented.

Acanthostega also drew attention to a formerly underappreciated part of early tetrapod anatomy: the inside of the lower jaw. Fish generally have two rows of teeth along their lower jaw, with a large number of small teeth on the outer row complementing a pair of large fangs and some small teeth on the inner row. Acanthostega showed that early tetrapods possessed a different dental plan: a small number of larger teeth on the outer row and a reduction in the size of the teeth populating the inner row--changes that probably accompanied a shift from feeding exclusively in the water to feeding on land or with the head above the water.

This insight enabled experts to recognize additional tetrapods among remains that had long sat unidentified in museum drawers. One of the most spectacular of these finds was that of a Late Devonian genus from Latvia called Ventastega. In the 1990s, following the discovery of Acanthostega, researchers realized that a lower jaw collected in 1933 was that of a tetrapod. Further excavation at the original Ventastega site soon yielded more material of exceptional quality, including an almost complete skull.

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The article is too long to post it all. I've posted the first two of seven sections. The rest is here:
Getting a Leg Up on Land.

[PatrickHenry]

Everyone be nice.

[PatrickHenry]

Evolution Ping The List-O-Links

A conservative, pro-evolution science list, now with over 320 names. See the list's explanation, then FReepmail to be added or dropped. To assist beginners: But it's "just a theory", Evo-Troll's Toolkit, and How to argue against a scientific theory.

[GarySpFc]

Here we seen another one of the millions of "Just so" fairy tales.

[muskah]

Have been pondering this evolution question for a while now. The tetrapods wern't the first to make this move. The arthropods beat us to it with as much or more diversity. So it has happened twice. Is the belief in a common aquatic vertebrate ancestor universally held, or could it have happened multiple times. Same question with inverts.

[sirchtruth]

In the almost four billion years since life on earth oozed into existence, evolution has generated some marvelous metamorphoses.

LOL! So, is this how agendized science is describing evo now?

The false profit must be very proud.

[Physicist]

Intermediate forms between fish and land tetrapods: another successful prediction of common descent (not to say Darwinism).

[Ken522]

What's readily apparent about this article is how the author presents "evolution" as a logical, simplistic, utterly natural progression in the development of species ... based on nothing more than wishful thinking; "Because that's the way things should be!"

[Physicist]

Here we seen another one of the millions of "Just so" fairy tales.

It is these transitional fossils that defeat your worldview, not the story. You're not fighting against the tale, but the tail.

[Rudder]

A little tale about Romer:

I attended a lecture at Berkeley given by Romer when he was in his 90's. The big Life Science's auditorium was packed. Romer hobbled up to the podium and began...

"When I last gave a lecture here, many, many years ago, I was greeted by an old friend and colleague (a colleague whose name escapes me). As we were walking to the auditorium on a beautiful summer's evening a huge tree limb from an old eucalyptus fell and killed him on the spot.

Romer stopped, took a drink of water, and began his lecture without any further comment about the incident.

It left us all scratching our heads...

[Physicist]

What's readily apparent about this article is how the author presents "evolution" as a logical, simplistic, utterly natural progression in the development of species ... based on nothing more than wishful thinking; "Because that's the way things should be!"

No, the wishfully thought, just-so story was the "mudskipper" hypothesis. People assumed that fish flopped onto land and later gained limbs, and thought "that's the way things should be". This view has now been dashed against the hard reality of the fossils. The history of life on Earth is discovered, not invented.

[AdmSmith]

You obviously read very fast, but died you really read the original article? you posted 11 minutes after the first post. ;-)

[AdmSmith]

From the article "For example, we know that tetrapods have lost all the bones that protect the gills in fish but that the genes that govern their formation are still present in mice, where they function differently. We have also ascertained that in the neck region, the biochemical pathways that preside over the development of limbs have broken down. Although biologists can easily induce extra limbs to grow on the flank of a tetrapod, this cannot be done in the neck. Something special happened when tetrapods first evolved a neck that prevented limbs from sprouting there."

Very interesting.

[wolfcreek]

Everyone be nice.

In other words, none of the usual eye-gouging and punching below the belt. FAT CHANCE!

[AdmSmith]

Freudian slip: but died you really read the original article? => but did you really read the original article? ;-)

[RoadTest]

How do you know that? There aren't any witnesses.

[ConsentofGoverned]

For those who consider Evolution theory as the only mechanism of the development of life, a few questions.

If DNA is information, and if information for life from the first cell to modern life evolved - is the information for all past life forms still in present DNA?

If not, why not?

How did information for structure of first life occur? that is evolution prior to life forms?

For me the simple reason "Evolution theory" is only part of the answer lies in the above questions.

[Alamo-Girl]

Thanks for the ping!

[AdmSmith]

is the information for all past life forms still in present DNA?

No, usually a specific organism has DNA that is mutated, recombinated, incorporated from other organisms, and some is deleted frome earlier versions. Naturally, the majority of the DNA is conserved from the parents.

[doc30]

I read this one in the print version. I din't know it was on the website so soon. This is a really good article. It's fascinating that the more we study fossils and their geologic context, the more we learn about how evolution worked to produce what we see.

[ConsentofGoverned]

" has DNA that is mutated, recombinated, incorporated from other organisms, and some is deleted frome earlier versions"
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>............
So in present day DNA, the DNA of first single cell life forms have long ago been deleted. The deletion mechanism only works in "evolved forms" as basic single cell forms still exist in rock, volcano plumes, that have never evolved over geologic time. (yet the 1st life form no longer exists?)This has always troubled me, that some life forms are immune to evolution forces over fantastic time periods. What does that imply?