THE ANIMAL KINGDOM BEGINS TO COLONIZE THE LAND
ABOUT 20 DECEMBER; ABOUT 970,000 METERS UP THE LINE
The one-celled life forms had blindly led the way. The fungi had unconsciously helped prepare the ground. The insentient plants had begun their sweeping act of colonization, and were everywhere laying the foundations of the ecosystems of the ever-changing planet Earth. Now, one of the other great acts in the drama of life’s history was beginning. Various forms of marine animal life began their invasion of the world’s landmasses, at first haltingly and unevenly. Then, with bodily modifications reinforced by reproductive success, they made their way inland. They ensconced themselves within the habitats being created by plant life, and in so doing altered the character of those habitats, setting off feedback mechanisms that would drive evolution in many different directions. The heterotrophs had arrived on the land, and their perpetual struggle to obtain nutrients now had a vast, new setting.
The first terrestrial arthropods were to be the founders of the greatest success story in this huge drama, the insect world that was to pervade the Earth’s landscapes. The other major group of invaders had begun their colonization with fish-like animals wandering on shorelines, unusual little organisms pulling themselves about awkwardly by means of fins that contained simple appendages. It may have happened many times, in many places. One of these fish-like species gave rise to the first line of true four-limbed land animals—the tetrapods. We aren’t yet sure which of the first tetrapods are ancestral to the four-limbed animals of the world today. There were many false starts and dead ends. But eventually, from one of these earliest of all amphibians (or perhaps from multiple lineages of them) more advanced varieties of amphibians evolved—and the conquest of the land began in earnest.
The Lobe-Finned Fish
Biologists now believe that the first vertebrates to pull themselves onto the land were descendants (or variations) of fish that were very successful and numerous in the oceans of the Devonian Period: the lobe-finned fish, which are today represented by one species of coelacanth and several species of lungfish. Lobe-finned fish and tetrapods are now considered to be a subclass of the vertebrates known by the name Sarcopterygii, reflecting the genetic relationship of the two groups. These fish have strong pelvic and pectoral fins. In contrast to other kinds of fish, their pelvic fins are connected to their main body by a femur and their pectoral fins are connected by a humerus.1
The genetic basis of terrestrial locomotion may have existed in lobe-finned fish as far back as 400 million years before the present. There are physical structures in these fish which are homologues (structures of similar type) of carpal (wrist) and tarsal (ankle) bones. Further, the extent of bone deep in the fins of late Devonian Period fish suggests structures which are analogous to the forearm. Recent research has revealed that key regulatory elements in the Hox genes that control the expression of autopods (the ends of limbs) in tetrapods can be found in such species as zebrafish and skate (rays). Researchers hypothesize that it was a change in the regulatory elements of Hox genes in Devonian fish that allowed for the evolution of limbs from analogous structures in the fin. Specifically, a change in the function of the Hox D gene may be responsible for the evolution of four-limbed animals.2 It must be emphasized that the transition from water to land involved more changes in body type than just the formation of limbs. There were concomitant changes to the structures of the ears, neck, and significantly, the brain case.3
The Transition from Fish to Tetrapod
It needs to be understood from the outset that the transition from certain species of lobe-finned fish to the tetrapods is an area of paleontology in which new discoveries are regularly altering our picture. It should also be understood that the evidence used to reconstruct the events of the sea-land transition is extremely difficult to come by and is often revealed to us in frustratingly incomplete, fragmentary form. But there are some aspects of this transition from water to land that we do seem to have firm evidence of. Tireless fossil searches and painstaking, thorough analysis have uncovered significant examples of Devonian Period lobe-finned fish that possessed anatomical traits that bear strong resemblances to those of the first tetrapods of which we know. Broadly speaking, the group of Devonian lobe-finned fish from which we think the tetrapods eventually evolved is known as the osteolepiforms (although the traditional definition of this taxon has come under criticism). A very thoroughly studied example of this group is known as Eusthenopteron. One of the world’s foremost paleontologists has closely examined a CT (computed tomography) scan of an example of this species and has concluded that its interior nasal structure is more tetrapod-like than fish-like. The skeleton of the pectoral fin has a structure which resembles an arm, with a humerus, radius, and ulna (although there is no structure that resembles a hand). In the words of the scientist analyzing the scan,
Eusthenopteron foordi is one of the most scientifically important fossil vertebrates. It is a fossil lobe-finned (Sarcopterygian) fish, which belongs in the stem group of the Tetrapoda or land vertebrates. This means that it is more closely related to land vertebrates – to us – than any living fish.4
What were the animals that were the nexus between the lobe-finned fish like Eusthenopteron and the first true tetrapods? The fossil evidence we actually have in hand seems to indicate that a variety of animals known as elpistostegids (which we will describe below) was this transitional group. Elpistostegids include those fish thought to be direct forbears of tetrapods and the earliest, most primitive tetrapods themselves. As far as when this occurred, before 2004 it was generally thought that the evolution of limbs (however rudimentary) from fins had taken place between 370 million and 360 million ybp. But in recent years new evidence has emerged, as we will see, and doubt about the role of the elpistostegids has arisen. Consequently, our estimate of when tetrapods evolved continues to be pushed farther back in time. And the role of fish like Eusthenopteron has become less clear as well.
There are several other intriguing finds that have been made in the search for the origins of the tetrapods. A lobe-finned fish named Panderichthys, an apparent elpistostegid, actually had digit-like structures in its pectoral fin, suggesting the origin of fingers.5 Further analysis indicates that Panderichthys possessed a humerus that appears to have features which are both intermediate between those of fish and tetrapods and features which are unique to it.6 The taxonomic position of this fish is still being elucidated, and it may be a sister group—derived from a common ancestor but branching off in its own direction—to the first tetrapods.7
The earliest tetrapod yet discovered that bore a physical resemblance to these fish and yet displayed unique traits is known as Acanthostega, another elpistostegid, regarded by many paleontologists (until recently) as the most primitive tetrapod known. It is definitely the early tetrapod about which we have the best fossil evidence. According to research done by Dr. Jennifer Clack of Cambridge University, the foremost authority on this animal, Acanthostega retains certain braincase structures resembling Eusthenopteron and Panderichthys, but its middle braincase, hyomandibula (the connection between the lower jaw and the skull) and the structures surrounding its ear regions are unlike them.8 Acanthostega (more specifically in this case, Acanthostega gunnari) may have been primarily aquatic. It had four limbs, the back two of which resembled paddles, and its limbs had eight digits.9 Its true place in the taxonomy of tetrapods is still being determined.
The discovery of a specimen that has been given the name Tiktaalik roseae, to which we referred in the chapter on life, altered the 370-360 million ybp chronology that had previously been postulated for the transition from fish to tetrapods. Tiktaalik was an elpistostegid, discovered in 2004 in the Nunavut Territory of northern Canada. Dated at 375 million ybp, it pushed back our estimates for the evolution of tetrapods. This animal was a mélange of fish and tetrapod characteristics. Like a fish it had scales and fins with webbing. But like an early tetrapod, its head was flat and it possessed a neck. (Necks are a crucial development in evolution. Fish have to turn their entire bodies to look at something. Land-based vertebrates can move their heads independently.) Tiktaalik also had structures which were homologous to the tetrapod arm and wrist.10 Detailed examination of the specimen showed that its arm and chest structures strongly suggest it had the ability to push its head up out of the water. Such a capability would have been of great utility in the shallow waters and mudflats in which we believe Tiktaalik lived.11 It seemed for a few years as if the decisive transitional animal had been discovered. But new findings have changed our picture.
Possible evidence of tetrapods that preceded Tiktaalik has been discovered in Poland. The evidence is a trackway found in an abandoned quarry. The trackway has been securely dated to around 395 million ybp, and appears to have been made in an area of shallow water, such as a tidal pool or lagoon. There appear to be tracks made by animals of various sizes. Some of the prints seem indicative of an aquatic animal pushing away with a single limb. The stride lengths and spacing of other prints show evidence of an animal that was capable of genuine, four-limbed locomotion—a tetrapod. The animal appears to have been capable of lifting its hindquarters off the ground in order to walk, which would indicate the presence of a sacrum. [The sacrum is a triangular structure of bone found at the base of the spine. Fish don’t possess this structure; land vertebrates do.] The tracks from the purported tetrapod indicate an animal 40-50 centimeters in length. There is also a track made by a smaller animal, one which may only have been capable of pushing itself along on two limbs, but the evidence in this case is not yet definitive. Further, a number of individual prints (those not found in a track-like setting) indicate an animal in excess of two meters in length, an estimate derived from the width of the print in comparison to other purported tetrapods or tetrapod-like animals.12
Several of the individual prints appear to show distinct digits. These prints are similar to those made by other animals, such as Ichthyostega [another early tetrapod] and Acanthostega. The age of the prints would appear to argue that tetrapods did not emerge from the elpistostegids, but rather that the two lines of animals coexisted for 10 million years. Moreover, it would seem to indicate that tetrapods evolved in tidal areas rather than inland, as has long been thought. As to why the fossil record seems to indicate that elpistostegids preceded tetrapods, the authors of the trackway study argue that a tidal pool evolutionary setting would make the preservation of fossils unlikely. It may be that elpistostegids colonized certain areas conducive to fossil preservation earlier than tetrapods. It would appear, the authors contend, that both the elpistostegids and the tetrapods must have considerable ghost lineages—lineages that must have existed, based on the phylogeny of the organisms being considered, but for which fossils have not yet been discovered. For elpistostegids they consider such a lineage must be at least 10 million years in length, and for tetrapods it must be at least 18 million years. They stress that further investigation and exploration to uncover fossil evidence is absolutely necessary.13
There are scientists who caution against trying to glean too much from this trackway evidence, but most of the paleontologists who have examined it find it persuasive. Of course, there are several possible ways this trackway can be interpreted:
- there were at least two fully evolved tetrapods that existed at 395 million ybp, one of which was the true ancestor of the modern tetrapods
- the tetrapods represented in the Polish trackway were an evolutionary dead-end and a line of elpistostegids indeed gave rise to the modern tetrapods
- the elpistostegids were an evolutionary dead-end and the examples of them we have found had no descendents
- neither the tetrapods found in Poland nor those elpistostegids that have been uncovered are the true ancestors of all modern tetrapods; we have yet to find the true ancestral group
None of the Polish evidence negates the significance of the finds that have been made. It is clear that the genetic basis of limbs existed in Devonian fish. It is clear that there was a fin-to-limb transition, and that several of the key species that have been identified show morphological evidence of this. It is clear that there were indeed fish with legs and lungs (see below). Sometime in the early Devonian we may suppose that an animal evolved which was ancestral to those tetrapods and the elpistostegids that have been uncovered. It seems very likely that these ancestral types propelled themselves at first on strong pairs of fins. They may have gulped air for its oxygen content. In short, even if we do not yet possess the full story, we know what the participants in that story had to be like. We know, broadly, how it happened.
The Evolution of Lungs
The evolution of limbs, new types of braincases, and necks were all essential elements in the adaptation of aquatic animals to the land. But in addition to those developments, the evolution of lungs in vertebrates was of key importance. It is probable that air-breathing mechanisms evolved independently several times in the bony fishes.14 Normally, fish have two means of acquiring oxygen. In most fishes, oxygenated water is moved through the gills by a process called buccal pumping (which involves flexing the cheek muscles to force water into the mouth). In others, such as many species of shark, oxygen is acquired through continuous swimming which forces water into the mouth and over the gills by sheer momentum (a process called ram ventilation). But certain fish have learned that by rising to the surface, they can take in water that has a higher oxygen content because of its direct contact with the air. This can be observed among fish that are in stagnant, poorly oxygenated water. In such a situation, the ability to gulp air and store gaseous oxygen in the body would have had obvious survival advantages, although there is an energy cost to the fish in having to swim to the surface, and it can make them more vulnerable to predators. But some fish can actually collect air in their stomachs, and can actually breathe air by absorbing it through their digestive tracts.15 Two biologists who have studied the pulmonary system extensively have hypothesized that lungs began as outpouches on the gut of certain fish, perhaps to facilitate oxygen storage. The selection pressure that they believe favored this adaptation was, as is so often the case today, aquatic hypoxia, in this case a reduction in oceanic oxygen levels. They postulate that the immediate ancestors of tetrapods were lunged animals that could gulp air for oxygen.16
From a genetic standpoint, evidence for the evolution of complex lungs points to the significance of genes that regulate the production of parathyroid hormone-related protein (PTHrP). It is this protein that appears to promote the process of alveolization (the forming of alveoli, the small, sac-like structures in which oxygen exchange occurs in the lungs), especially, as we will see, in mammals, and proteins similar to it are found across a wide spectrum of life forms. The evolution of surfactant (see below) is tied to PTHrP as well. There appears to have been a combination of selection pressures that affected the evolution of complex lungs, particularly fluctuations in atmospheric and oceanic oxygen levels, and variations in temperature. Increased surfactant production in early vertebrates would have facilitated the ability to survive environmental variations, which would in turn would have reinforced the growth of lung complexity, reinforcing the production of surfactant—a biological synergy of great significance.17
Surfactant is absolutely essential to lung function. Surfactant chiefly consists of phospholipids and specialized proteins. Surfactant facilitates lung function by reducing surface tension in the lung. Excessive surface tension would cause there to be less surface area with which to absorb air. Surfactant facilitates the passage of air into the lungs’ tissues, allowing them to expand more readily. There is evidence that surfactant-type substances evolved even before lungs did, and that Sarcopterygian fishes and tetrapods evolved a distinct variety of it. The importance of this may have been enormous. As the two pulmonary specialists have put it:
The tetrapod surfactant is much more surface active and may have enabled the development of more complex lungs with smaller respiratory units and a greater total respiratory surface area, paving the way for the occupation of land. It is possible that fish surfactant is a "protosurfactant" that evolved into tetrapod surfactants but was retained as a protective lipid lining for the gas bladders in the modern fish and in gas-holding structures that are not used for respiration.18
The significance of the lung’s evolution was enormous. There is evidence that the development of lungs and the increasing complexity of the heart were deeply interrelated. The complexity of the heart in turn facilitated the increasing complexity of the liver, an organ that produced and stored chemical energy necessary for the support of the brain, which evolved broad regulatory functions. The development of complex kidneys as an element of blood pressure regulation was influenced as well. In the broadest sense, metabolism, locomotion and respiration all co-evolved, which is to say that the evolution of each of these phenomena influenced the evolution of the others.19 Once again, the pervasive influence of synergistic processes can be seen.
The Radiation of the Amphibians
Fossil evidence of the amphibian radiation has been very difficult to unearth. There is a substantial fossil record of Devonian amphibia (comparatively speaking), and then there is a gap (from about 359 to about 345 million ybp) in the record during the early Carboniferous Period, what some paleontologists refer to as “Romer’s Gap” after a paleontologist who tried to discover amphibian fossils from this time period.20 There have been some important finds made. There were significant numbers of amphibian specimens unearthed at Nyrany, in what is now the Czech Republic, in the last decades of the 19th century. These specimens, dating from the Carboniferous, included examples of a very small amphibian known as Branchiosaurus. Also found at Nyrany were Anthracosaurs, a distinct kind of amphibian with features that suggest that it was part of the line of amphibians that eventually evolved into reptiles.21 Three body impressions of amphibians, dated at approximately 330 million ybp, (from the Mississippian Epoch of the Carboniferous Period) have been discovered in Pennsylvania, in the United States. The impressions, preserved in sandstone, show head, limb, and trunk outlines. There are also samples that contain footprints.22 A 300 million year-old fossilized amphibian skull was discovered in western Pennsylvania and announced in 2010. The animal was apparently predominantly terrestrial, indicative, perhaps, of an adaptation to a drier, cooler climate.23 Other finds have been made, but in general the search for ancient amphibians has been a challenging one.
The Arthropods Invade
Of crucial significance for the general ecology of the biosphere was the evolution and dispersal of the insects. Insects play an absolutely essential role in the Earth’s environment, as we will see in greater detail in a subsequent volume. Insects are a part of the vast Phylum Arthropoda, the origins of which stretch back to the Precambrian Eon. The arthropods preceded the first lobe-finned fishes who ventured ashore, and so we must count them as the earliest animal life on land. Various kinds of arthropods invaded the land independently many times. We have the first fossil evidence of arthropod terrestrialization in the Early to Mid-Ordovician, from about 488 to 460 million ybp. Evidence of the first true insects does not appear until the Early Devonian, beginning at about 416 million ybp.24 This was the start of the dominance, in terrestrial multicelled life, of the most spectacularly successful body plan in the Kingdom Animalia—head, thorax, and abdomen, accompanied by six legs. The first insects were wingless. The evolution of insect wings occurred by a process that has not yet been fully explained, but in the tropical forests of the Carboniferous Period large winged insects flourished. Indeed, many insects in the Carboniferous displayed very large sizes, basking in the warmth, moisture, and rich oxygen levels of Carboniferous tropical forests. (See below.) Indeed, the high oxygen levels in those regions may have facilitated the development of insect flight.25
The Carboniferous Period and the Continuing Consolidation of the Earth’s Landmasses
As noted earlier, at the end of the Devonian and the beginning of the Carboniferous there was glaciation in the far south of Gondwanaland. By the Late Carboniferous, approximately 310-300 million ybp, this glaciation was generally quite extensive, covering much of what would later be Antarctica, India, Australia, southern Africa, and other far southern regions. The majority of the world’s land remained south of the Equator in the Late Carboniferous. What would later be South America and Africa were still firmly attached, and Laurussia, which contained elements of North America and Russia, lay on both sides of the Earth’s midsection. What would ultimately become parts of China, Siberia, and Kazakhstan were the major landmasses of the northern hemisphere. The trend of consolidation that would bring about the formation of the biggest supercontinent in the Earth’s history, Pangea, was well advanced.26 The ocean that had existed between Gondwanaland and Laurussia had closed, and the collision of the two landmasses had produced a major mountain range, part of which was the tropical Appalachians, which in this period had already attained a height of more than 3000 meters. In many of the tropical latitudes it is thought there were major stretches of abundant rainfall and extensive areas of lush vegetation. Swamps are believed to have been widespread. Analysis of the fossilized plant samples from these regions indicates almost continuous growth, uninterrupted by dry spells. The result was a phenomenon known as the coal swamps—the regions where dense, decomposing vegetation formed the major coal deposits of the world. Large areas of what would later be the eastern part of the United States, the western regions of Europe, and the Donets Basin of Ukraine and Russia were covered in coal swamps during this period.27 It was this combination of climate, terrain, and land distribution that laid the foundations of the human ability to exploit coal for energy—and the dangerous, back-breaking, often lethal work required to extract coal from the ground.
Modifications in the bodies of fish that allowed for locomotion and respiration facilitated the vertebrate invasion of the land. Amphibians were eventually to find a place in a surprisingly wide variety of habitats. Insects pervaded these habitats, evolving an extraordinary variety of forms. The Earth’s climate continued its fluctuations, variations that can only truly be seen by stepping back and surveying millions of years in time. The continents continued their inconceivably slow drift, their collisions driving up mountains and altering the climates of whole vast regions of the Earth. A particular kind of amphibian began to evolve the ability to reproduce outside of the water, penetrating ecological niches until then unoccupied, and then, in the reciprocal manner of life, creating new kinds of environments. The age of the reptiles was at hand—and along with it, the appearance of a similar life form from which the mammalians—our class—evolved. The story is turning in interesting new directions, but our role in it is still many days away.