Aquagenesis : The Origin and Evolution of Life in the Sea

by Ellis, Richard (Author)

• ISBN13: 9780142001561
• ISBN10: 0142001562
• Edition: Reprint
• Format: Paperback
• Copyright: 2003-02-01
• Publisher: Penguin Group USA

Summary

Life on earth began in the sea, and in this tour de force of natural history, authority on marine biology and illustrator Richard Ellis chronicles more than three billion years of aquatic history. From the first microbes and jawless fishes that evolved into the myriad species we know today-sharks, whales, dolphins, and, of course, humans-Ellis reveals the deep evolutionary mysteries of the sea. Encyclopedic in scope and complemented by more than sixty drawings, Aquagenesisis a fascinating work that will astonish readers with the wonder, richness, and complexity of the evolution of life. "Quite simply, the best account we now have of the origins of human life." (Te Christian Science Monitor)

Table of Contents

Preface and Acknowledgments	p. vii
Introduction	p. 1
Early Days	p. 9
The Origin of Life in the Vents	p. 9
Water, Water Everywhere	p. 16
Here We Come, Ready or Not	p. 20
From Fossil to Squid	p. 25
The Garden of the Ediacara	p. 25
Wonders of the Burgess Shale	p. 31
The Cambrian Explosion	p. 40
What Happened to the Trilobites?	p. 44
Living Fossils	p. 51
Here Come the Mollusks	p. 54
Brachiopods	p. 55
Cephalopods	p. 57
The Vertebrates	p. 77
What Did it Mean to have a Backbone?	p. 78
The Jawless Fishes	p. 82
The Lungfishes	p. 87
The Coelacanths	p. 90
Pisces Triumphant	p. 96
The Evolution of Bioluminescence	p. 102
Sharks, Etc.	p. 112
Land Ho!	p. 130
Return to the Sea	p. 139
Marine Reptiles	p. 139
Encased in Armor Plate	p. 143
The Sea Snakes	p. 151
The Crocodilians	p. 159
Birds that Swim	p. 166
Marine Mammals	p. 175
The Sea Cows	p. 176
The Sea Weasels	p. 182
Flippers for Feet	p. 186
The Desmostylians	p. 197
The Cetaceans	p. 202
The Living Whales	p. 224
Everybody Back into the Water	p. 243
The Aquatic Ape	p. 243
The End of the Beginning	p. 256
References	p. 269
Index	p. 293
Table of Contents provided by Ingram. All Rights Reserved.

Excerpts

Chapter One

EARLY DAYS

THE ORIGIN OF LIFE IN THE VENTS

There may be a place on Earth--albeit somewhat inaccessible--where we might be able to see conditions not unlike those that the earliest life forms saw. Where the sea floor pulls apart (a phenomenon known as "sea-floor spreading"), cracks or rifts are created in the crust of the Earth. This activity usually, but not always, takes place along the mid-ocean ridge, a 40,000-mile-long undersea series of mountain ranges that snakes around the planet like the seams of a baseball, and is the largest single geological feature on the Earth's surface. The rifts mark the edges of the lithospheric plates, which bear the continents in their inexorable movement on the first 100 to 250 kilometers of the surface of the planet. The plates such as those of North America and Africa drift over the less rigid athenosphere, much like giant icebergs in the ocean. The formation of the plates occurs at the mid-ocean ridge, and they are consumed at "subduction trenches," where one plate slides under the lip of another. Where plates interact, earthquakes occur, volcanoes erupt, and mountains are pushed up. All three were manifest in November 1963, when the Icelandic island of Surtsey was born in a spectacular cataclysm of fire, lava, and steam. The rifts caused by the separation of the plates fill up with lava that wells up from within the earth, flowing outward from the center and moving across the ocean floor. As described by J. R. Heirtzler and W. B. Bryan (1975), "Bizarre as the idea seemed at first, it was becoming evident that the mid-ocean-ridge system was nothing less than a vast unhealed volcanic wound."

    The unique conditions around subterranean hydrothermal vents make them strong candidates for a deep-ocean location of the origin of life. Discovered by scientists aboard the research submersible Alvin in 1977 at a depth of 8,200 feet in the Galápagos Rift Zone of the eastern Pacific, hydrothermal vents are cracks in the seafloor at the juncture of two tectonic plates. Volcanic activity beneath the plates releases hot gases and dissolved minerals into the ocean, and heats the water to temperatures of nearly 700° F. At these vent sites--subsequently discovered along many other mid-ocean ridges--minerals are spewed into the water in clouds known as "black smokers" that eventually dissolve and disperse into the water column. In the vicinity of these vents, a completely unknown fauna was discovered, living not on oxygen, as every other known life form does, but on hydrogen sulfide, a substance that is poisonous to most living creatures. These chemo-synthetic life forms (as opposed to those that are photosynthetic, i.e., able to process sunlight), include 6-foot-long tubeworms with red, feathery plumes but no mouth and no gut; football-sized, snow-white clams with blood-red innards; ghost-white crabs; yellow mussels; floating "dandelions" that are related to the jellyfishes; and eyeless shrimp with light-detecting organs on their backs.

    Discovered in 1985 at the 11,000-foot-deep Trans-Atlantic Geotraverse (TAG), the 2-inch-long shrimp Rimicarís occur in dense schools in the immediate vicinity of hydrothermal vent sites. They have no eyes (their scientific name can be translated as "rift shrimp without eyes"), but on their dorsal surface there is a pair of organs just below the skin that is light-sensitive. Since they live in total darkness, the ability to "see" is probably unnecessary, but these optical organs may be useful in detecting the faint light emitted by the vents. Like many other hydrothermal vent animals, rift shrimps do not breathe oxygen, but subsist on sulfides dissolved in the water or scraped off the sides of the mineral stacks.

    Because the internal digestive tube (trophosome) of the 6-foot-long tubeworm Riftia pachyptila has no means of ingesting particulate food matter, their feeding mechanisms baffled scientists until it was discovered that the trophosome was colonized by vast numbers of sulfur-oxidizing bacteria. "We recognized," wrote Childress, Felbeck, and Somero (1987), "that the bacteria and Riftia had established what is known as an endosymbiotic relation":

The Riftia -bacteria endosymbiosis is mutualistic. The tube worm receives reduced carbon molecules from the bacteria and in return provides the bacteria with the raw materials needed to fuel its chemolithoautotrophic metabolism: carbon dioxide, oxygen, and hydrogen sulfide. These essential chemicals are absorbed at the plume and transported to the bacteria in the trophosome by the host's circulatory system. The worm's trophosome can be thought of as an internal factory, where the bacteria are line workers producing the reduced carbon compounds and passing them along to the animal host to serve as its food.

Almost everything about the vent tubeworms is special. They must transport poisonous materials through 6 feet of tissue to reach the deeply embedded symbionts; to facilitate carbon dioxide flux from inside to outside they must maintain pressures of dissolved CO ₂ approaching that of carbonated beverages. They can grow as much as 3 feet in their first year, a growth rate that surpasses that of any other marine creature. It is not known how tubeworms and other vent animals are able to colonize hot springs, but the larvae might ride the hydrothermal vortices to new sites that they identify by chemical cues.

    The rift clam Calyptogena magnifica and the mussel Bathymodiolus thermophilus also depend on chemosynthetic endosymbiosis, but they have resolved the problem somewhat differently. In the clam, the bacteria are not internal but reside in the gills, where they can obtain oxygen and carbon dioxide from the water. The basic metabolic plan is the same as that of Riftia : the bacteria oxidize the sulfide and supply the clam with fixed carbon compounds. "Like other invertebrates harboring sulfur bacteria as endosymbionts Calyptogena has a greatly reduced ability to feed on and digest particulate foods" (Childress et al. 1987). Each of the symbiont-containing animals is colonized by a host-specific strain of bacteria; even though the job all these bacteria do is similar, each bacterium has adapted itself to just one species.

    These creatures of the hydrothermal vents flourish in a pitch-black, superheated, sulfide-rich environment without any connection whatever with sunlight; they are as far removed from life as we previously understood it as life on another planet. There may have been a fortuitous combination of elements and conditions in the primeval ocean that more or less accidentally created "life." Jack Corliss, one of the discoverers of the Galápagos rift animals (1977), was among the first to suggest that life might have originated in conditions similar to those found in the vents. (Corliss, who was then at Oregon State University, became obsessed with the possibility of the vent systems as the source of life, and left the university to devote himself full time to working on the problem. In 1991, he had moved on to become chief scientist of Biosphere 2, the closed-system environment built in the Arizona desert, and he is now affiliated with the Central European University of Budapest.) Working with Corliss and John Baross, Sarah Hoffman, a graduate student, formulated a theory that life had originated in the Archean Period, about 4.2 billion years ago, on the sea floor, which was probably much more hydrothermically active than it is today. The authors suggested that the water issuing from the Archean vents was so hot that it cracked the molecular bonds of the rocks, and released carbon and carbon compounds such as methane into the solution. Then simple organic molecules formed out of the newly formed chemical elements, and while some of them rose into the water column, some adhered to the rock faces, and formed a clay that provided a safe haven for these molecules, giving them the opportunity to form more complex organic molecules. Out of this jumble of molecules, argued the authors, "biopolymers" could be formed, producing fragmentary nucleic and amino acids, which, in a system far from equilibrium, could organize themselves into new forms, i.e., primitive living cells. In 1985, Baross and Hoffman wrote that the hydrothermal activity "provided the multiple pathways for the abiotic synthesis of chemical compounds, origin, and evolution of `precells' and `precell communities,' and ultimately, the evolution of free-living organisms."

    In 1988, Stanley Miller and Jeffrey Bada refuted the suggestions of Corliss, Baross, and Hoffman, claiming that "the proposal for a hydrothermal-vent origin of life fails each of the three proposed steps involved in the origin of life." The debate about the origin of life--certainly one of the most intriguing problems in all of science--continues, but it is fascinating to consider at least the possibility that the hydrothermal vents, unknown and unsuspected until the 1970s, might provide some clues. In a 1991 article in the journal Eos , John Baross wrote:

Important new discoveries on the properties of the early earth and atmosphere, including the frequency and size of bolide impacts, have strongly implicated submarine hydrothermal vent systems as the likely habitat for the earliest organisms and ecosystems, while stimulating considerable discussion, hypotheses and experiments related to chemical and biochemical evolution. Some of the key questions regarding the origins of life at submarine hydrothermal vent environments are focussed on the effects of temperature on synthesis and stability of organic compounds and the characteristics of the earliest organisms on Earth. There is strong molecular and physiological evidence from present-day mircoorganisms that the earliest organisms on Earth were capable of growing at high temperatures (about 90°C) and under conditions found in volcanic environments.... Further molecular and biochemical characterization of the presently cultured thermophiles, as well as future work with the many species, particularly from subsurface crustal environments, not yet isolated in culture, may help resolve some of the important questions regarding the nature of the first organisms that evolved on Earth.

    Coincident with the earliest traces of life on Earth, 3.9 to 3.8 billion years ago, there are indications of a particularly intense bombardment of extraterrestrial bodies here and on the moon. In a 2000 article, David Kring of the University of Arizona wrote that impact events certainly affected life on Earth--an example is the Chicxulub impact 65 million years ago, which signaled the end of the dinosaurs; they also might have "provided the necessary environmental crucibles for prebiotic chemistry and the evolution of life." During a period that is believed to have lasted as long as 200 million years, the moon was hit at least 1,700 times, and "the number of impacts on Earth would have been an order of magnitude larger, implying >10,000 impact events" (Kring 2000). According to Mojzsis and Harrison (2000):

Recent explorations of the oldest known rocks of marine sedimentary origin from the southwestern coast of Greenland suggest that they preserve a biogeochemical record of early life. On the basis of the age of these rocks, the emergence of the biosphere appears to overlap with a period of intense global bombardment. This finding could also be consistent with the evidence from molecular biology that places the ancestry of primitive bacteria living in extreme thermal environments near the last common ancestor of all known life.

    "Commonly," wrote David Kring, "this is interpreted to mean that life originated (or survived the impact bombardment) in volcanic hydrothermal systems. However, during the period of bombardment, impact-generated hydrothermal systems were possible more abundant than volcanic ones. The heat source driving these systems is the central uplift and/or pools of impact melt. In the case of a Chicxulubsize event ... melt pools may have driven a hydrothermal system for 10 ⁵ yr. The dimensions of these systems can extend across the entire diameter of a crater and down in depths in excess of several kilometers." Kring is saying that life either began in hydrothermal vent systems that were generated by impacts, or perhaps life originated in some other fashion, and was best suited to withstand the cumulative effects of this concentrated extraterrestrial bombardment.

    Everett Schock of Washington University in St. Louis believes that life began in an environment similar to that of today's hydrothermal vents. In 1992, in the journal Origin of Life and Evolution of the Biosphere , he published the article "Chemical environments of submarine hydrothermal systems," in which he noted that the necessary building blocks for life--methane and ammonia, for example--are not available on the Earth's surface, but that the conversion to organic compounds from carbon dioxide and carbon monoxide could have taken place in the sea, especially in the presence of high temperatures. Schock also believes that the early atmosphere of Earth would have been unwelcoming to life because of the constant bombardment by ultraviolet radiation, but deep in the ocean, as hydrogen sulfide spewed from cracks in the sea floor, it mixed with seawater to provide the chemical energy for the synthesis of life.

    All cells are surrounded by membranes, which are composed of fat (lipid) molecules. It is difficult to imagine a cell that lacks a membrane, so the question arises: Which came first, membranes or nucleic acids? The answer, says John Howland (2000), is probably "both":

First, imagine a hot spring on the ancient seafloor. Heated water emerging from the vent would have already percolated through the rocks and clay of the seabed and would be charged with a variety of dissolved substances as well as suspended clay particles. These particles would transport bound molecules that are precursors of life, including amino acids and nucleotides, as well as small polymers formed from them: polypeptides and polynucleotides. In addition, there would probably be lipid molecules present, some of them likely being soluble in hot water, but insoluble in cold. Then, according to the scenario, the hot water would cool on mixing with seawater, and the lipid molecules would precipitate from solution, forming spherical vesicles. If some fraction of those vesicles contained some of the suspended clay particles inside, with their burden of bound amino acids, nucleotides, and so on, the stage might be really set. The raw materials for protein and nucleic acids would be internal, in close proximity to the clay surfaces, which could catalyze the formation of protein and nucleotide polymers. And a lipid membrane would tidily surround the whole business.

    At the 1992 Nobel Symposium, "Early Life on Earth," held in Karlskoga, Sweden, participants presented papers on virtually every aspect of this fundamental and complex subject, which were then collected into a volume published by Columbia University Press (Bengston 1994a). In "Vitalists and Virulists: A Theory of Self-Expanding Reproduction," Günter Wächtershäuser argued that Pasteur was correct about the impossibility of spontaneous generation in maggots, but wrong when he stated that the generation of a living organism from chemical compounds is impossible. (Wächtershäuser also said that the idea of an "RNA World" requires "an extreme food specialist--more extreme than any heterotroph known today.") He believes that the only possible way for life to have begun is autotrophically, and that "the first organism is a chemoautotroph. It uses the formation of pyrite from hydrogen sulfide as a source of electrons and as its energy source."

    With Claudia Huber, Wächtershäuser began testing his idea that a process on the deep ocean floor could transform basic inorganic chemicals into organic chains, the building blocks of life. In 1997 they wrote:

The origin of life requires the formation of carbon-carbon bonds under primordial conditions. Miller's experiments, in which simulating electrical discharges in a reducing atmosphere of CH ₄ , N ₃ and H ₂ 0 produced an aqueous solution of simple carboxylic acids and amino acids, have long been considered as one of the main pillars of the theory of a heterotrophic origin of life in a prebiotic broth. Their prehistoric significance, however, is in question, because it is now thought that the primordial atmosphere consisted mostly of an unproductive mixture of CO ₂ , N ₂ , and H ₂ 0, with only traces of molecular hydrogen.

    Huber and Wächtershäuser (1998) converted amino acids into their peptides "under anaerobic, aqueous conditions. These results demonstrate that amino acids can be activated under geochemically relevant conditions. They support a thermophilic origin of life and an early appearance of peptides in the evolution of a primordial metabolism " (my italics). Metallic ions in sulfides (such as iron pyrites, readily available in seafloor rocks) interact with the carbon- and hydrogen-rich gases that belch from the hydrothermal vents, creating what Huber and Wächtershäuser call the first organic molecule: acetic acid (CH ₃ CO ₂ H), a simple combination of carbon, hydrogen, and oxygen, best known for giving vinegar its pungent odor. The authors believe that the formation of acetic acid is a primary step in metabolism, those chemical reactions that provide the energy for cells to manufacture the necessary biological ingredients for life. They theorize that around the vents, catalytic metallic ions enabled acetic acid to form, and this catalyzed the addition of a carbon molecule to produce the three-carbon pyruvic acid, which reacts with ammonia to form amino acids, which then link up to form proteins.

    Wächtershäuser's ideas about the origin of life are among the very few that can be tested in the laboratory, and now George Cody and his colleagues at the Carnegie Institution of Washington's Geophysical Laboratory have done just that. Wächtershäuser calls his theory "the iron sulfur world theory," because he believes that metallic surfaces, particularly those in iron sulfide, "would have been promising facilitators or catalysts that created the precursor chemicals of living cells" (Wade 2000). Cody et al. (2000) placed iron sulfide samples in 24-carat-gold capsules and then subjected the capsules to temperatures of 250°C and pressures equivalent to 2,000 atmospheres. This produced large amounts of pyruvates, in addition to a hydrogen sulfide-like substance that resembled the H ₂ S produced by volcanoes above and below the sea. Their experiments demonstrated that pyruvates could be synthesized under the proper conditions, and they wrote, "The natural synthesis of such compounds is anticipated in present-day and ancient environments wherever reduced hydrothermal fluids pass through iron sulfide-containing crust. Here, pyruvic acid was synthesized in the presence of such organometallic phases. These compounds could have provided the prebiotic Earth with critical biochemical functionality."

    Of Wächtershäuser's theory, the biochemist Michael Adams of the University of Georgia said that he "had developed a very reasonable and testable hypothesis for the origin of organic material relevant to life" (Wade 2000). For the issue of Science (25 August 2000) that featured the research paper by Cody et al., Günter Wächtershäuser wrote an essay that he called "Life As We Don't Know It." He said:

It is occasionally suggested that experiments within the iron-sulfur world theory demonstrate merely yet another source of organics for the prebiotic broth. This is a misconception. The new finding drives this point home. Pyruvate is too unstable to ever be considered as a slowly accumulating component in a prebiotic broth. The prebiotic broth theory and the iron-sulfur world theory are incompatible. The prebiotic broth experiments are parallel experiments that are producing a greater and greater medley of potential broth ingredients. Therefore, the maxim of the prebiotic broth theory is "order out of chaos."

    The final chapter of Cindy Van Dover's The Ecology of Deep Sea Hydrothermal Vents (2000) is "Vent Systems and the Origin of Life." Van Dover, an Alvin pilot from 1989 to '91 and now assistant professor of biology at the College of William and Mary, reviews many of the suggestions presented above, and writes,

(Continues...)

Excerpted from AQUAGENESIS by Richard Ellis. Copyright © 2001 by Richard Ellis. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.