It is common knowledge that the human body consists of about 65% water. People cannot live any longer than five days without H20. Individuals of all ages love to sail the oceans, swim in the sea and soar under or speed across the waves. It comes as no surprise, then, that some part of the human psyche remembers millions and millions of years ago before animals came on shore. What is still questionable is how or why these animals made the move from water to land. The journal articles discussed below give some of the latest findings on this topic.
Early in the Devonian Era, close to 400 million years ago, all the continents were grouped closely together and surrounded by the seas. The climate ranged from dry weather to torrential rains as some tropical areas do today. Even flowers had not yet evolved on land, let alone vertebrates. Many of the sealife were preparing for that next big step onto land with lung-like organs that would later evolve into swim bladders to control buoyancy. Some of these creatures moved on lobed fins or fleshy appendages that supported their weight while crawling underground. In time, they adapted to terrestrial life and evolved into amphibians with fully developed legs.
In what kind of environment did the transition to lobed fin first occur? This has recently been a “bone” of contention. Marine biologists Graham and Lee understand that air-breathing fishes may be seen as possible models for the Paleozoic evolution of vertebrate air breathing and the transition to land. They note how recent studies suggest that marine air-breathing amphibious fish in tropical, high intertidal zone habitats are analogs of early tetrapods and that the intertidal zone are feasible early habitats for the Devonian land movement by vertebrates.
However, in response to such scientists, Graham and Lee argue that selection pressures imposed by life in these intertidal zones are insufficient to have led to the necessary respiratory capacity or break from water required for the vertebrates to move to land. The marine amphibious fishes, which occur mainly on rocky shores or mudflats, have reached what the authors call “their land-penetration” limits and remain linked to water by their respiratory structures that are less efficient in air and more vulnerable to desiccation than lungs. Such fish definitely cannot succeed in the biologically complex terrestrial environment that awaits them on the seashore. They are just too tied to the water, even as adults.
Because of the proximity of reefs and mudflats to dry land, as well as the exposure to tidal cycles and wave action, fish that live in these habitats will sometimes occasionally be exposed to air. It is not surprising then that intertidal fish tolerate air exposure. In fact, as scientists as Graham have seen previously, activities by these fish include laying eggs in the upper intertidal splash zone, riding waves to feed at the upper limits of an algal ridge, developing mud turrets or territorial boundary walls, and feeding with the moving tides.
Along with these activities come a variety of specializations regarding respiration, vision and terrestrial locomotion. However, even though they do spend some of their time breathing air, their link to the water has not decreased. For example, during high tide, one of these fish called the mudskipper revert back to water burrows to remoisten their skin and respiratory systems. Further, their reproductive systems are closely tied to the water. Besides serving as a refuge, the burrow serves as a nursery for developing eggs. Lee and Graham also stress that other physiological changes also weigh against these animals in terms of being able to move onto land. It is highly likely, they say, that earlier forms of intertidal marine life would have developed lungs.
Graham and Lee conclude, therefore, that “The unique combinations of changing habitat, exploitable land resources, and the breathing and locomotory capacities of the ancestral tetrapods drove selection for a group of organisms that could, in effect, come farther onto the shore. In the absence of extensive environmental change, this evolutionary tape cannot be replayed for the modern amphibious marine fishes.”
How, then, did the first transition develop from water to land? Scientists know that sometime during the Devonian period the harsh sun caused severe droughts. Fish would have been trapped in drying pools and faced death. To survive, a few eusthenopterons must have dragged themselves on their fins, as mudskippers do today, out of the puddles in search of deeper water. Some could then have evolved on land. Their fins became legs, they grew five fingers and toes, they started to walk. They became tetrapods, the ancestors of all four-legged animals today. The fossil of this ancient ancestor, according to Erik Jarvik, is believed to be ichthyostega.
Ichthyostega was a relatively large, about 4 feet, with a stout body. Although it was originally considered to be the transitional form between fishes and Carboniferous amphibians, its skull possess several primitive, fish-like features. The spine of Ichthyostega is notochordal, instead of being based on a series of loosly jointed but interlocking vertebrae. It most probably did not have internal gills, its tail bore fish-like supraneural spines, and it had a massive ribcage with thoracic ribs that were long, flattened bones that overlapped with other ribs to protect the body from being injured. The large pelvis was attached to the spine, the hindlimbs were significantly smaller than the forelimbs and most likely functioned more like paddles than legs. The knee was flexible without evidence of an ankle joint. It had seven toes.
Since Jarvis’ find, other scientists such as Jennie Clack have developed some ideas about similar early tetrapods that have been uncovered. She found, for example, another species of Devonian tetrapod called Acanthostega. Although different from Jarvik’s fossil, it clearly shared the same ancestry. The forelimb of Acanthostega had eight digits instead of seven. Both came from the Upper Devonian of East Greenland, similar to the only other known Devonian tetrapod limb, Tulerpeton from Russia, which has six digits.
Clack and others such as Coates questioned if these tetrapods were meant to walk, why did they have different numbers of digits and why were they paddle shaped? The conclusion: The limbs may have been adaptations to an aquatic rather than a terrestrial environment. During this period of time, instead of walking on firm land, the tetrapods had to move around in swampy wet environments. The pattern of digits also altered the proposed model for limb development where digit number is unspecified, rather than earlier models that are rejected because they postulate a fixed number of elements in the ancestral limb. Clack and Coates thus challenged pentadactyly (five fingered) as primitive for tetrapods. The form of these limbs rather suggested early specialization in the evolution of the tetrapod limb bud.
Such findings led to a widely agreed-upon transition in which tetrapods diverged from their lobe-fin ancestors sometime during the Frasnian, and were broadly dispersed throughout tropical and subtropical localities by the end of the Devonian. Most importantly, the Devonian tetrapods appear to have been mostly if not exclusively aquatic. In other words, they were like fishes with legs.
Although Clack had significantly advanced the idea of the early tetrapods, scientists were still concerned about what they called the Tournaisian Gap, or about 20 million years that yielded very few fossils between the latest Devonian tetrapods and a very wide variety of primitive aquatic, secondarily aquatic and terrestrial tetrapods from the Middle and Upper Visean. In 2002, Clack looked once again at a skeleton found in 1971. She realized that it was not a fish as thought, but actually another tetaprod. This named Pederpes finneyae, at least functionally pentadactyl, was really the first to show changes for land locomotion. With a later found American Whatcheeria, it represented the next most primitive tetrapod after those of the Late Devonian. Thus, it bridged the gap that previously existed between Late Devonian and mid-Carboniferous tetrapods.
Pederpes finneyae and the other similar tetaprods were not actually made for walking. Paleontologists believed that most of them plodded along very slowly for millions of years and did not pick up the pace until about 210 million years ago. A skeletal reconstruction of the first four-legged land animal that crawled onto land suggests that it was not very comfortable in its new setting as it crept or scuffled wormlike along.
As noted, even though it was some sort of amphibian, Pederpes finneyae’s bone structure was more similar to a fish. Yet it had very strong shoulders and hips that could support the body’s weight without the buoyancy of water. Scientists propose two ways that it may have moved: Perhaps it did a type of forward walk with the body held rigid and the limbs moving in alternating diagonal sequence from front left and hind right and front right and hind left. The muscular forelimbs would have bent elbows, but the hind limbs would appear more flipper-like, so the pelvic region dragged along on the ground. Or, perhaps this unique animal moved along similar to an inchworm, pulling its hips and back legs upwards and then pushing its back upward while moving its front legs forward.
About 20 million years later, according to Patton, Paton, Smithson, Clack, rose another tetrapod called Casineria kiddi. The earliest tetrapods lacked hands that could flex, as humans curl their fingers and toes because they have a notch in the flexor surface on the phalanges. Due to this, walking on a rocky land, that necessitates the ability to curl the paws around various obstacles, would have been difficult. Acanthostega and Ichthyostega could only bend their hands very little amounts. Thus, while they did actually have hands, they were just slightly evolved hands. The evolution of Casineria kiddi, led to these notches on each phalange.
Casineria also had other adaptations to life on land. Its vertebrae, for instance, connected to one another to create quite a stiff backbone that probably served as a means to hold up the animal’s body weight. The earlier Devonian tetrapods, however, had a much flimsier backbone that was similar to those of fish. It offered considerably less support. Further, Casineria’s humerus or upper arm bone bore a shaft in the middle and flared out at each end. The stubby humerus of the Acanthostega did not narrow at the middle as bones commonly seen today. Casineria’s proportionately lengthier and more thin and shapely form would have held the creature up better and aided it while walking.
Finally, the evolutionary advances travel to the very end of the limbs where Casineria had a set of five digits on each hand and foot. This established the fundamental model that runs through the rest of vertebrate years to the tips of human fingers. With finds like that of Casineria, paleontologists have begun to fill in some of the missing evolutionary blanks between the Devonian swamps and the amniotes of the Carboniferous period.
These tetaprods are important because their structure can still be seen today. However, they were in fact not the first animals to make the transition from water to land.
A fossil discovery a few years back found that 80 million years before the dinosaur, about 290 million years ago, there may have been a reptile running around on two legs. The 25-centimeter-long herbivore Eudibamus cursoris is the earliest known vertebrate able to scurry on its hind limbs. The find suggests that bipedalism may be more common than believed, but not necessarily a definite route to evolutionary success (Stokstad).
Like the raptors in Jurassic Park these animals ran fast on their toes. The vertebrate found in Germany had hind limbs that were 64% longer than its forelimbs and 34% longer than its trunk, proportions similar to those of modern two-legged lizards. Feet with long digits would have given the animal considerable stride ability. In addition, its tail helped it move quickly. Muscles attached to the tail could have made Eudibamus’s hind limbs powerful enough for two-legged sprinting. It also helped to keep the animal’s center of gravity close to its hip that is essential for balancing a two-legged gait. Further, the early vertebrate had evolved a new form of knee joint, which let it run with its feet directly beneath its body (Stokstad).
In other vertebrates of that time, the legs jutted outward from the body, since one of the paired shinbones or tibia connected with the underside of the mostly horizontal thigh bone or femur, while the other shinbone or fibula attached to the end of the femur. In Eudibamus, however, both shinbones fit onto the end of the femur and formed a hinge-like joint that put all of the leg in one plane as in dinosaurs and humans. It gave the animal an energy-efficient posture that allowed the bones, as well as the muscles, to help support the animal’s weight. Even though it may not have been the most delicate bipedal animal, it had great strides over the four-legged competitors. Unfortunately, this did not help the animal in the long run (no pun intended). Like other animals that developed two legs on the evolutionary line before the dinosaurs, birds, and mammals, it did not stay around long (Stokstad) and was a dead end in the scheme of things.
Once these animals started coming up on dry land, how did they survive without water, especially amphibians that have thin skin and can easily dry out? According to zoologists as Lillywhite, scientists have found frogs throughout the world that use a variety of different means to keep themselves wet. For example, the South American painted belly monkey frogs, Phyllomedusa sauvagii, secrete waxes from specialized glands in their skin and then wipe themselves with the wax. They sort of “massage” the wax all over their body from the top of their heads to the tip of the limbs. Wax frogs in other arid parts of the world have somewhat similar behavior.
Evolution is an ongoing process, and even today there are groups of animals in the very process of moving from the sea up onto the land. One of these is the crabs, another group of arthropods that are distantly related to the insects. Crabs are basically marine animals, but like the insects have a high probability for eventually adapting to the land. In Japan, for example, crabs are presently making the transition from marine to freshwater life, which can be seen as a first stage on the road to the land. All along the coast there are various species of crabs that live in the intertidal zone. When the tide is out, they run freely across the mud and rocks. They can spend significant amounts of time on land by keeping a reserve supply of water inside their carapace. Meanwhile, like the insects, these crabs’ hard exoskeleton provides some protection from the outside world.
Clack, J.A. “An Early Tetrapod from Romer’s Gap.” Nature (2002) 418: 72-76. [electronic version]
Clack, J.A. “From Fins to Fingers.” Science 304.5667 (2004): 57-59. [electronic version]
Coates, M.I, and J.A. Clack. “Polydactyly in the Earliest Known Tetrapod Limbs”
Nature. (1990) 347: 66-69. [electronic version]
Graham, Jeffrey B., and Heather J. Lee. “Breathing Air in Air: in what ways might extant amphibious fish biology relate to prevailing concepts about early tetrapods, the evolution of vertebrate air breathing, and the vertebrate land transition?.” Physiological and Biochemical Zoology 77.5 (2004): 720-732. [Electronic Version]
Jarvik, E. “The Devonian tetrapod Ichthyostega.” Fossils and Strata, (1996) 40: 1-213.
Lillywhite, Harvey B. “To wipe and wax: in dry times, some frogs employ a curious method for saving water. Natural History 110.10 (2001): 58-64. [electronic version]
Paton, R.L., T.R. Smithson, and J.A. Clack. 1999. An Amniote-like Skeleton from the Early Carboniferous of Scotland. Nature 398: 508. [electronic version]
Stokstad, Erik. “First Upright Vertebrate Lived Fast, Died Young” Science 290.5493 (2000): 917. [electronic version]
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