Aquagenesis: The Origin and Evolution of Life in the Sea  – ★★★★
“That we live on land is, in the grander scheme of things, best regarded as an anomaly, or even an eccentricity – albeit with sound evolutionary justification. The story of life is, if we retain a true sense of proportion, a story of life at sea“(Philip Ball)
After I read Monarchs of the Sea by Danna Staaf last year, I wanted to read a deeper work on this topic and chose Aguagenesis by marine biologist Richard Ellis. The author aims to demonstrate how life originated in water some 3.9 billion years ago, what species evolved first in water and why, what species followed them and how evolution changed courses multiple times with various animals choosing to dwell on land next and then returning to waters. Richard Ellis starts his book by discussing the origin of water itself and a 2 inch-long shrimp-like creature without eyes capable of subsisting on hydrogen sulphide alone, which is poisonous to most living creatures, before talking about more complex and diverse marine life that roamed the oceans in the final stages of the Cretaceous period, some 65 million years ago. “More than 99% of all the species that have ever lived on Earth are [now] extinct” [Ellis, 2001: 22], says the author, and that makes that extinct life even more fascinating, especially in what it can tell us about the diversity of life and our own, human, origin. This book may be on an academic side and now a bit dated, but it is still a perceptive and engaging account of the mysteries that still surround the evolution of life in the sea.
Richard Ellis starts his story from the very beginning, speculating that life began in some “pitch-black, superheated, sulphide-rich environment without any connection whatsoever to sunlight” [Ellis, 2001: 11]. These conditions were not dissimilar to those found today in subterranean hydrothermal vents deep in the ocean. Ellis then discusses very primitive life forms that first emerged at that time. The period is called Ediacaran, and at the Burgess Shale, for example, evidence was found of ediacara biota. Such organisms encompass soft-bodied entities and fauna prevalent at that time and included dickinsonia, an organism that evolved 635 million years ago and which represents a symmetrical ribbed oval whose precise functions and are still unknown; and cyclomedusa, a circular fossil. Ellis states that that “ediacarabiota populated the ocean up until the end of the Precambrian 535 million years ago, with the close of the Precambrian much of the ediacara biota became extinct” [2001: 31]. This period was before the “greatest evolutionary event in Earth’s history: the Cambrian explosion” [2001: 32].
When the “Cambrian explosion” happened some 541 million years ago, simple animals began to be succeeded by many complex ones, and, in that period, we start to see animals that had jaws, shells, limbs, bones and teeth. The author here asks some thought-provoking questions such as – will conscious life ever had a chance to appear if the ediacaran fauna not died out and prevailed? In the Cambrian period, first cephalopods, molluscs, crabs, nautiluses (or “living fossils”), trilobites and other ammonites appeared. There is still a debate why the “Cambrian explosion” of life occurred, and the most prevalent theory has to do with an increase in oxygen levels. Thus, first vertebrates appeared, but the precise origin of vertebrates is also shrouded in mystery [Ellis, 2001: 80]. In the Ordovician period, such creatures emerged as armoured Sacabambaspis or jawless fish, and first cephalochordates.
The Devonian period was an era of fish. “Fishes have been swimming on Earth for more than 450 million years. They predated the dinosaurs by hundreds of millions of years, and they were the first creatures to have an internal skeleton. In that sense, they are the ancestors of all vertebrates – amphibians, reptiles, birds, and mammals. Fishes are among the most successful animals ever” [Richard Ellis, 2001: 96]. The author states how the development of their jaws was a very important evolutionary advantage. Through their jaws, fish were able to prey on other species and the development of their “armour” helped them against predators. Dunkleosteus is probably best known armoured fish of the Devonian period and could reach the length of 9 metres. Here it was interesting to read about the discovery of bioluminescence in fish or its the ability “to glow” in the dark. Starfish, certain sharks and squid also have this ability. Some of them are able to glow because of the luminous bacteria that give off light. These bacteria cannot exist without its host [Ellis, 2001: 103]. The function of this light may include attracting mates, indicating sex or luring preys.
In the Permian period, some 298 million years ago, ancestors to many today’s sharks appeared and at least the very first “sharks” looked nothing like sharks today, the author says. For example, dental arrangements of these sharks sometimes appeared on their heads or backs [Ellis, 2001: 299]. For example, helicoprion was a shark who had a very unusual, spiral teeth arrangement embedded in the lower jaw. Precursors to amphibians also appeared, including labyrinthodonts, living some 359 – 251 million years ago. Another section of the book is titled “Return to the Sea Marine Reptiles” and here the author says: “reptiles evolved on land, but after their dispersal to various terrestrial habitats (becoming eventually either dinosaurs or lizards in the process), some returned to the sea” [2001: 260]. Thus, first sea turtles, snakes and crocodile ancestors are discussed. It was particularly interested to read that “crocodilians breezed through the cataclysmic K-T Extinction (66 million years ago), which eliminated some 70 percent of all living animals….but now face extinction threat from humans” [Ellis, 2001: 266]. Nowadays, Philippine crocodile and Siamese crocodile are some of the most endangered crocodiles on earth (there are about 250 endangered Siamese crocodiles left in Cambodia).
In discussing marine mammals (sea otters, seals, whales) and their antecedents, it was interesting to discover some puzzles in the evolution of whales. Ellis states that the oldest whale fossil dates to the Middle Eocene, 49 million years ago, but whales may have existed long before that date, and that present-day whales have probably evolved from such creatures as indohyus, a mouse-deer-like walking creature living some 40 million years ago and from mesonychus ambulocetus, living 55 million years ago in the Eocene epoch. The main question is, of course, why did early cetaceans take to water? This is still a mystery. The rest of this chapter talks about the mysteries of echolocation, biological sonar, especially in dolphins, why it evolved and what evolutionary paths might have led to it.
Fun begins when the author starts discussing speculations that the man might have had purely aquatic ancestors. In the 1970s-1980s, Elaine Morgan proposed a view, inspired by marine biologist Alister Hardy, that humans may be descendants from aquatic creatures because they are radically different from other mammals, for example, they display relative lack of body hair, have a power of speech and are bipedal like penguins. The theory is that “there was a period during which early hominids lived a semi-aquatic existence; they were never aquatic like cetaceans, but, rather, went in and out of the water frequently” [Ellis, 2001: 247, 267]. However, one question still remains: why certain mammals returned to the sea and others did not?
Aquagenesis is perhaps a book more on the evolution of sea life, rather than its origin, but the big plus of the book is that it does not dumb down its content like so many other non-fiction books do nowadays. Besides, it introduced me to many other related interesting books, such as to John Long’s The Rise of Fishes: 500 Million Years of Evolution, John McCosker’s Great White Shark, J.G.M. Thewissen’s The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea and to Richard Fortey’s Trilobite: Eyewitness to Evolution. Overall, Aquagenesis is a thought-provoking book which discusses one fascinating topic, inspiring you to know more.
(when you buy from my website I earn a small commission, so if you like what I do, you can support me this way)