The story of life on Earth is long, spanning a period of about 3.5 billion years. In a short book review such as this, only the "headlines" can be reported.
How did it all begin? Life, that is.
The prevailing theory is that the first primitive organisms developed in the deep seas near hydrothermal vents. These are extraordinary places in Earth's ocean crust that have cracked open, allowing molten magma to seep out. Sometimes called, "black smokers," they glow as red lanterns in the darkness of water too deep to receive sunlight from above. Black fumes rich in minerals pour out the vents, adding a deeper blackness to already black waters. The primitive life that emerged here several billion years ago harvested the energy in rotten-egg-smelling hydrogen sulfide (H2S). The book of Biogenesis of The Bible According to Einstein depicts this wondrous moment in bio-history when the first progenitor forms of life appeared:
The air was warm and moist.
The ground was wet and warm.
And everywhere was fog.
over the dark waters of the Earth;
valleys held cradles of white fog;
and the mountains were clothed in clouds.
Black and hardly visible was the land.
Steam floated above the ocean.
The Earth was in a foggy cloud.
And darkness was upon the face of the deep.
For millions of years, it was thus so.
And the winds began to lightly blow and swirl the fog. And the clouds moved about in mysterious ways, as white patches swirled around the peaks of mountains. The mist collected in the valleys and floated among the hills . . .
. . . And the wind blew and moved the elements, stirring the waters there below. And at the bottom of the shallow waters, mud, sand and earth were mixed. As a finger of the fog moved in circles, it seemed to stir the waters and mud too. And different molecules in mud were merged. And the molecules combined in many ways. And new molecules emerged.
And for millions of years, the muddy waters turned and molecules were mixed and made.
And one day near a hydrothermal vent rich in minerals, organic matter containing hydrogen, oxygen and carbon atoms combined to form a complex molecule. And the complex molecule captured the chemical energy of its parent molecules. And it pulsated like unto a tiny heart. And it breathed the energy of life, vibrating several seconds under its own power.
Even today, the hydrothermal vents are a feeding ground for exotic forms of sulfur-eating microbes that form one of the three fundamental branches of life, Archaea. The other two branches are Bacteria and Eukaryotes.
Once life began, it stayed and proliferated.
So life began about 3.5 billion years ago. Eventually more sophisticated organisms developed in an Earth without oxygen in its atmosphere, an atmosphere rich in carbon dioxide and unbreathable, indeed poisonous, to humans. The oxygen in the atmosphere has been mostly biologically generated: For example, countless cyanobacteria breathed in CO2 and exhaled O2 for hundreds of millions of years. Plants, when they settled on the continents, did the same. As a result, the CO2 in the atmosphere has decreased while the O2 has steadily gone up. Initially, the oxygen released did not accumulate in the atmosphere because oxygen is highly reactive, combining, for example, with metals to make rust.
The fossil record in the Archaean Eon (2.5 billion years ago and earlier) is scant: There are some microfossils in the Fig Tree chert of Transvaal, South Africa and in the Pilbara rocks of Western Australia. During this eon, some microbes in warm shallow waters organized themselves in matted layers that form dome-like structures called stromatolites, testaments to nature's great building ability. Although the picture of early life is fragmented, one thing is clear. Once life began, it stayed and proliferated.
Stromatolites were built everywhere during the Proterozoic Eon (2.5 billion years ago to 570 million years ago). Today, their remains can be found in the Canadian Shield that extends from the Great Lakes to the Hudson Bay.
There were two great biological developments during the Proterozoic: eukaryotic cells and sexual reproduction. The former probably arose from symbiosis, an arrangement of two species for their mutual benefit. There are many examples of symbiosis in Nature. Flowers provide nectar for insects, while insects pollinate the plants. Human digestion (and for that matter, almost all mammal digestion) depends on microbes in intestines to breakdown food. In the Proterozoic, only primitive life forms existed, but undoubtedly some of them "got together" for their mutual benefit. An organism efficient in photosynthesis might have lived with an organism efficient in enzyme production. Eventually, they fused to create a more sophisticated life form. In this way, the nucleated cell, or eukaryote, developed. In such cells, components called organelles perform various functions and DNA is housed in a nucleus. Eukaryotic cells are the building blocks for one of the three fundamental branches of life, a branch that includes everything from fungi and slime molds, to plants and animals.
Sexual reproduction, which first appeared about a billion years ago, accelerated the pace of evolution. Previously, changes in species arose mainly through mutation, a process that was relatively infrequent, quite haphazard and often destructive rather than beneficial. Sexual reproduction allowed offspring to inherit the features from their parents, thereby allowing change through combination.
By the start of the Cambrian Period (570 million years ago), macroscopic creatures had appeared: centimeter-sized marine worms, jellyfish, tubular and disk-shaped metazoa. Life exploded in a variety of diverse, sophisticated forms in what is called the Cambrian radiation. (For more on this, see Jupiter Scientific's review of Stephen J. Gould's book Wonderful Life. With all the basic anatomical parts in place, life progressed to more complex forms and managed to occupy new ecological niches. Primitive jawless fish encased in plates appeared in the Ordovician about 500 million years ago. Plant life exited the oceans during this period followed by worms during the Silurian (438 - 408 million years ago), only to be joined by arthropods (which became insects) and tetrapods (which became amphibians) during the Devonian (408 - 360 million years ago). Thus, the "conquest" of continents began. Before life had invaded land, the lands of Earth had been a dustbowl, a waste land not so different from the surface of the present-day Moon.
and make a toast to our existence.
Continuing the saga, fish flourished in the seas; plants and insects proliferated on land; and tetrapods evolved. "Soon," the first true forests on Earth appeared. During the Carboniferous (360 - 285 million years ago), Earth possessed thick vegetation and lush forests full of insects and life including reptiles, which had evolved from amphibians. In the air flew the first flying insects, primitive forms of dragonflies. During the Permian Period (285-245 million years ago), the continents came together to create one giant land called Pangaea. Reptiles diversified and the progenitors of mammals arose. The Permian ended in the greatest extinction of life of all time, during which 95% of marine species disappeared forever.
When an extinction event takes place, it is a great opportunity for life's survivors. Indeed, this was the case for the Permian-Triassic extinction. Reptiles "seemed to seize" the "moment" to become the ruling class of Earth. In fact, the Mesozoic (245 - 65 million years ago) is known as the era of the "ruling reptiles." Not only did the dinosaurs dominate Earth's land, but also pterosaurs filled the skies, and swimming reptiles occupied the seas. The story of the dinosaur is a familiar one, which need not be repeated here. It suffices to say that these giant (and not-so-giant) reptiles prevailed for more than 150 million years, only to be extinguished in a cataclysmic event, an asteroid that struck near the Yucatán Peninsula. Not only did dinosaurs die but many other life forms also did so. At this point, birds (which had evolved from coelurosaur-like dinosaurs about 150 million years ago via Archaeopteryx) and, figuratively speaking, mammals "seized" the opportunity to "take control" of Earth. During the Cenozoic Era (65 million years ago to the present), the mammals, which started out rather small, grew larger, diversified and developed in complexity. Among the mammals were primates, and the among the primates eventually would appear apes, monkeys, australopithecines and man. So here we are today. Perhaps now is the time to lift the wineglass and make a toast to our existence.
Of course, so many developments have been left out here: species that came and went, sometimes dominating for countless millennia. And then there were great lost families of life such as trilobites, conodonts, graptolites, extinct microscopic foraminiferan and spherical radiolarian protozoa, ammonoids in swimming shells, defunct exotic echinoderms, ancient extinct brachiopods, the dinosaurs, etc. -- the list would fill a thousand tomes.
Life: A Natural History of the First Four Billion Years of Life on Earth by Richard Fortey describes evolution's great saga in more detail than given here. Richard Fortey is a senior paleontologist at the Natural History Museum in London, and there is a very "British" feeling to his writing. The book contains some nice black and white photographs of fossils, living creatures and expedition sites. But the best parts of Life are also its drawback. The history of life is often interrupted with wonderful narrative accounts of expeditions of the author or others. These parts are particularly enjoyable to read as one relives the experiences of a naturalist, adventurer or paleontologist. One also learns a lot about how field work is undertaken. However, these narrations and historical anecdotes usually interrupt the main goal of the book: to relate the story of life. The book also does not always go in chronological order, which is sometimes confusing. In regard to life's history, the reader sometimes finds herself/himself asking, "Now where are we?" Nevertheless, Life is excellent. If you would like to learn the history of life from an expert, then read this book.
Life: A Natural History of the First Four Billion Years of Life on Earth is available through the internet at Amazon.com. Click here to order. Jupiter Scientific participates in Amazon.com's Associates Program.