Diverse, bilaterally symmetrical groups of organisms, each being a lineage or branch on the Tree of Life, appear to have emerged within a few million years during which myriad biophysical relationships coalesced. A study of these relationships reveal patterns of anatomical and physiological diversification, as well as ecological strategies for feeding, which indicate that major animal lineages diverged tens of millions of years before their first appearance as fossils.
As with all systems, the cumulative effects of minute biophysical changes remained obscure for a time, the lag period, until the macroevolutionary threshold was crossed, establishing the developmental toolkits for gene pools above the level of species during the Cryogenian. (“Cryogenian” is from the Greek cryos “cold” and genesis “birth” and is a geological period that lasted from 850 to 635 million years ago.) The Cryogenian was the precursor to the Ediacaran—a geologic period that lasted from 635 to 541 million years ago, and was named after the Ediacara Hills of South Australia.
It was during the Ediacaran that the ecological success of multicellular animals (termed “metazoans”) was achieved. Within this time frame, the various animal lineages diverged and evolved along their own paths, apparently “gathering steam” for a major evolutionary diversification during the Cambrian. (The Cambrian period was named after Cambria, the Latin name for “Wales,” and lasted from 542 to 488 million years ago.) This burst of evolutionary diversification is called the “Cambrian explosion,” during which most of the major animal phyla (the 6th highest level of scientific classification out of 9) appeared in the fossil record. (“Phylum” is from the Greek phylon, a “race” or “stock,” as in “phylogeny,” from the Greek phylon “race” + geneia “origin,” from genes “born.”)
The Cambrian explosion involved the creation of complex, self-reinforcing feedback loops of biophysical potential within the existing ecological context, including the oxygenation of the ocean’s waters. Genetic regulatory networks governed the establishment and interactions of metazoan (multicellular animals) feedback loops long before they became ecosystems, as we know them today. The lag period in the macroevolution of animals, such as that preceding the Cambrian explosion, also underpinned the evolution of plants. On the one hand, plants and animals evolved more diverse cellular chemistry during these lag periods, whereby to regulate their basic genetic responses to environmental change. The Cambrian explosion, on the other hand, is about behavioral changes among the animals themselves due largely to the evolution of the classic predator-prey relationships.1
To appreciate the dynamics of early evolution in both animals and plants requires an understanding of the processes that generate biodiversity and the expansion of ecological networks through the various scales of geologic time. One such expansion was the transition from an aquatic lifestyle to a terrestrial lifestyle.
African lungfish walk and bound along the bottoms of their watery home on their slender, whip-like pelvic fins. Their aquatic habitat is non-constraining to a variety of means for getting around because water is buoyant and thus alleviates the tug of gravity. Moreover, the lungfish of today are related to some of the earliest, four-legged terrestrial vertebrates, or “tetrapods,” from the Greek tetrapodedon, “on all fours,” which corresponds to the Latin quadruped, “four-footed.”
Around 416 million years ago, during the Devonian geologic period (416 to 360 million years ago), certain species of bony fish—called the lobe-finned fishes for their thick, muscular, fleshy fins—began to evolve such features as larger limbs with digits, which are boney structures analogous to your fingers and toes. In addition, the evolution of air bladders found in fish may have helped them transition from their oceanic habitat onto land. Lobe-finned fishes are credited with the evolutionary step that led to the amphibians, thus making lobefins the ancestors of all four-limbed land vertebrates—amphibians, reptiles (including dinosaurs), birds, and mammals. The fossils of these remarkable creatures come from the red rocks of Devon, a large county in southwestern England, hence the name “Devonian.”2
Amazingly, some lobe-finned fish are still around today, such as the coelacanth (pronounced SEAL-a-canth), which has survived deep in the Earth’s seas vritually unchanged for millions of years. The first known coelacanth was caught in a deep-water gill net set for sharks about six hundred feet down off the mouth of the Chalumna River in southeastern Africa. In December 1938, Marjorie Courtney-Latimer, curator of a museum of natural history in East London, South Africa, went to the docks looking for interesting fish among the day’s catch. There she found a 119-pound, lobe-finned fish that she described as, “the most beautiful fish I had ever seen . . . a pale mauve blue with iridescent silver markings.” Professor J.L.B. Smith described the fish as a new species in 1939 and named it Latimeria chalumnae in honor of Courtney-Latimer and for the Chalumna River.
Upon examination by scientists, it was dubbed a “living fossil” because the remains of such creatures had been discovered only in rocks more than seventy-five million years old. At that time, the individual represented the only surviving species of coelacanths—a lineage of lobe-finned fishes that originated some 380 million years ago in the latter part of the in the Devonian geologic period. It was thought to have become extinct, however, in the Upper Cretaceous period, around 80 million years ago, which is the date of the youngest fossil.
How could this lineage of fishes have survived all that time without leaving a trace of its existence? A species can seem to disappear for three reasons: they are genuinely rare, they live in an uncommon habitat, or their remains do not fossilize well. In the case of coelacanths, all three reasons seem to apply, especially the latter two. They inhabit the “twilight zone” between five hundred to eight hundred feet deep in waters adjoining steep, rocky slopes of volcanic islands, where they cluster together in submarine lava deposits during the day. In this kind of habitat, sediment seldom settles fast enough to preserve a carcass.
Species that are typically low in numbers of individuals achieve their persistence through an array of biophysical mechanisms. The one employed by the coelacanths is reliance on restricted “hot spots” of especially favorable habitat in which the local rate of growth is almost invariably strongly positive when the population is not crowded. Indeed, these ancient fish are rigidly adapted to a couple of narrowly specific habitats, both of which are now threatened with drastic modification that may well cause the coelacanths to disappear into the great mystery from whence they came.
In the dance of survival, the coelacanth has five ominous strikes against it: there are just two surviving species of a taxonomic group that was once considerably richer; it has not changed in millions of years; it is adapted to a specific habitat now threatened by human-caused pollution and human intrusion, such as severe pressure from local fishermen; it has a narrow resource base; and it has a poor ability to disperse.
Since 1938, however, other coelacanths have been caught in deep water off the Comoros Islands, which lie between the coast of southeastern Africa and the northwestern tip of Madagascar. And on September 18, 1997, the wife of Mark Erdmann, the author of an article about coelacanths in Indonesia, saw one in Sulawesi (Celebes), Indonesia, being wheeled across a fish market on a cart. She barely had time to photograph the fish before it was sold.
Then, on July 30, 1998, Sulawesi fishermen dragged up a 4 1/2-foot-long, sixty-five-pound coelacanth that they had caught in a gill net set for sharks about four hundred feet down off the young volcanic island of Manado Tua in north Sulawesi. This specimen turned out to be a new species, and was named Latimeria manadoensis (manado refers to the island and ensis means “belonging to”). Manado Tua is known to have submarine caves at about the same depth as those on the Comoros Islands, six thousand miles away.
Today, all coelacanths are deemed to be endangered and are thus protected by the Convention on International Trade in Endangered Species of Wild Flora and Fauna. The reason for this status is the small population, an estimated five hundred individuals around the Comoros Islands, coupled with the low rate of reproduction (coelacanths bear live young). In the final analysis, however, we humans are the ones threatening the coelacanths’ very existence, primarily through the chemical pollution and acidification of their deep-sea habitat.
The continued survival of the coelacanth, after 380 million years in the deep sea, is suddenly threatened by major changes in its environment. These changes have been created by an upstart, invasive species that has been around for a mere five to eight million years—us.
What does it say about us, the human species, if we destroy the biophysical integrity of the coelacanth’s habitat and its patterns of self-maintenance to the point of its extinction? It means that a whole, major line of evolution will suddenly disappear—forever. It means that all living individuals in the species, each the culmination of—at minimum—a 380-million-year chain of unbroken genetic experiments, will cease to be. How will the ocean ecosystem change with the loss of the coelacanths and their biophysical function as part of the system? Although to me such a pointless loss is unconscionable, it’s just one more chapter in the never-ending story of change—the constant universal process of ever-novel outcomes and thus total irreversibility of either a deliberate decision or a reaction to an unplanned circumstance.3
AND LIFE MOVED ONTO LAND
The actually transition from an aquatic lifestyle to colonizing land required four major steps. The first is moving form a water-borne creature to an bottom dweller, which it termed “terrestriality.” Next came the evolution of limbs with digits. Third was solid, substrate-based locomotion, where the creature could move about on the seafloor. And fourth was the creature’s ability to alternate its gaits by using pelvic appendages to elevate itself enough to propel its body forward. Finally, these characteristics needed to be synchronized so an animal could use its limbs in alternating fashion, like the movement in walking, while employing the seafloor as a solid surface from which to use its pelvic appendages as a means of forward propulsion.4
A Devonian creature, dubed “Tiktaalik” (pronounced tik-TAA-lik), was discovered in 2004 on Ellesmere Island in Nunavut Territory of the Canadian Arctic. Better known as a “fishapod” (fish-footed), Tiktaalik is a 375-million-year-old fossil fish that has a crocodile-like head and strong, bony fins. Its fins not only have thin ray bones for paddling, like most fishes have, but also have sturdy interior bones that would have allowed Tiktaalik to prop itself up in shallow water and thus use its limbs for support to move about, as most four-legged animals do. The fossil remains also show a mixture of traits belonging to both fish and amphibians.
Although Tiktaalik lived about 12 million years before the first four-footed creatures appeared toward the end of the Devonian period, some 363 million year ago, it has a combination of features that show the evolutionary transition between swimming fish and their descendents, which includes amphibians, dinosaurs, birds, mammals, and of course, humans. (Tiktaalik, an Inuktitut word meaning “burbot,” a freshwater fish related to true cod, has been chosen as the generic name of an extinct lobe-finned fish that shows additional characteristics of terrestrial animals, such as ribs, a neck, and nostrils on its snout for breathing air.)5
The end of the Devonian heralded the beginning of the Carboniferous period (named for the rich deposits of coal in England) that dates from about 359 million years ago to approximately 299 million years ago. A relatively warm climate, oceans populated by now-extinct marine reptiles, and land occupied by dinosaurs characterize this geologic period. At the same time, however, new groups of mammals and birds, as well as early of flowering plants, accompanied the dinosaurs. Biologically, one of the greatest evolutionary innovations of this geologic period was the amniotic egg, which is compartmentalized with a liquid-filled sac wherein the embryo develops, a sac for food, and a sac for biological wastes, that gave the ancestors of reptiles, birds, mammals the ability to lay their eggs on land without fear of desiccation.6
AND TERRESTRIAL ECOSYSTEMS ARE BORN
Many of the most amazing events in the history of the Earth and of life occurred during the Cretaceous (derived from the Latin creta, “chalk”), which lasted from approximately 145.5 to 65.5 million years ago. It was during this time that stable continents first began to grow, albeit the process took about a billion years. In addition, the first abundant fossils of living organisms appeared, mostly bacteria and what are called extremeophiles (lovers of extreme conditions.) As well, dinosaurs both great and small moved through forests of ferns, cycads, and conifers and flying dinosaurs and birds piled the currents of air.
Moreover, the Cretaceous saw the first appearance of many life-forms that would evolve into the ecosystems that would one day serve us—the human species. Perhaps the most important of these events, at least from the perspective of terrestrial life, was the appearance of the flowering plants, technically called “angiosperms.” Coincidentally, many groups of modern insects were beginning to diversify, the oldest known being ants and butterflies. Aphids, grasshoppers, and gall wasps also appear, as well as termites in the later part of this period. Another important insect to evolve was the social bee, which proved to be an essential part of the ecology and evolution of flowering plants—and thus the various terrestrial ecosystems of today.7
Series on Biodiversity:
1. The foregoing discussion is based on: (1) Douglas H. Erwin, Marc Laflamme, Sarah M. Tweedt, and others. The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals. Science 334 (2011):1091–1097; (2) Susan Millus. Biology’s Big Bang Had a Long Fuse. Science News 180 (2011):14; (3) Urbilaterian. http://en.wikipedia.org/wiki/Urbilaterian; (4) Ediacaran Period. http://en.wikipedia.org/wiki/Ediacaran; and (5) Cryogenian. http://en.wikipedia.org/wiki/Cryogenian.
2. (1) Heather M. King, Neil H. Shubin, Michael I. Coates, and Melina E. Hale. Behavioral Evidence For the Evolution of Walking and Bounding Before Terrestriality In Sarcopterygian Fishes. Proceedings of the National Academy of Sciences USA 108 (2011):21146–21151 and (2) Nick Bascom. African Lungfish Walk In Water: Study Suggests Four-Legged Locomotion Began At Sea. Science News 181 (2012):12.
3. The preceding discussion of the coelacanth is based on: (1) Quoted in Sid Perkins, “Back from the Dead?” Science News 172 (2007):312–314; (2) J.L.B. Smith, “A Living Coelacanthid Fish from South Africa,” Nature (1939):748–750; (3) Susan L. Jewett, “The Coelacanth: More Living Than Fossil,” Natural History Highlight of the Smithsonian National Museum of Natural History. 2003. http://www.mnh.si.edu/highlight/coelacanth/; (4) Daniel Goodman, “How Do Any Species Persist? Lessons for Conservation Biology,” Conservation Biology 1 (1987):59–62; (5) Karen Hissmann, Hans Fricke, and Jürgen Schauer, “Population Monitoring of the Coelacanth (Latimeria chalumnae),” Conservation Biology 12 (1998):759–765; (6) Janne S. Kotiaho, Veijo Kaitala, Atte Komonen, and Jussi Päivinen, “Predicting the Risk of Extinction from Shared Ecological Characteristics,” Proceedings of the National Academy of Sciences USA 102 (2005):1963–1967; (7) Peter Forey, “A Home from Home for Coelacanths,” Nature 395 (1998):319–320; and (8) Mark V. Erdmann, Roy L. Caldwell, and M. Kasim Moosa, “Indonesian ‘King of the Sea’ Discovered,” Nature 395 (1998):335.
4. Heather M. King, Neil H. Shubin, Michael I. Coates, and Melina E. Hale. Behavioral Evidence For the Evolution of Walking and Bounding Before Terrestriality In Sarcopterygian Fishes. op. cit.
5. The preceding two paragraphs are based on: (1) Devonian Period. http://science.nationalgeographic.com/science/prehistoric-world/devonian/; (2) Tiktaalik. http://en.wikipedia.org/wiki/Tiktaalik; (2) Tiktaalik roseae. http://tiktaalik.uchicago.edu/meetTik.html ; and (3) What has the head of a crocodile and the gills of a fish? http://evolution.berkeley.edu/evolibrary/news/060501_tiktaalik.
6. (1) The Carboniferous Period http://www.ucmp.berkeley.edu/carboniferous/carboniferous.php; and (2) Introduction to the Amniota. http://www.ucmp.berkeley.edu/taxa/verts/amniota.php.
7. The Cretaceous Period. http://www.ucmp.berkeley.edu/mesozoic/cretaceous/cretaceous.php; (2) Cretaceous. http://en.wikipedia.org/wiki/Cretaceous; and (3) Mosè Rossi, Maria Ciaramella, Raffaele Cannio, and others. Extremophiles 2002. Journal of Bacteriology, 185 (2003):3683–3689.
Text © by Chris Maser 2012. All rights reserved.