Life of Earth

"Your cells contain not only the genetic instructions for making a human body but also old genes that the cells of your evolutionary ancestors used. These old instructions are crammed into your chromosomes like old memorabilia in an attic."

"Almost all generic diversity is in bacteria, archaea, and protists. The fungi, the plants, and all the animals are only a subset of the variation within the protist group."

"There is about as much genetic difference between a bacterium in the soil and an archaean in a hot spring than there is between a bacterium and you."

"Even the little bit of wobbling that Earth does experience has been enough to cause about twenty ice ages during the last two million years of Earth's history. We have the moon to thank for the relative stability of Earth's movements."

"Two hundred fifty million years ago, at the end of the Permian period, volcanoes in what is now Siberia erupted for about two hundred thousand years, producing layer after layer of lava; the deposits are called the Siberian Traps. The sulfur dioxide from these eruptions caused acid rain, and the carbon dioxide caused global warming, extensive enough to have at least initiated the greatest mass extinction in Earth's history: the Permian extinction wiped out about 95 percent of species. Biodiversity recovered after this event, but the recovery took almost one hundred million years."

"The origin of life itself was a lucky accident. When the oceans formed on an Earth no longer molten, conditions were hellish. ...scientists call this the Hadean period, named for Hades...Within a few hundred million years, these oceans were filled with living cells that resembled living bacteria. ...the kind of sheer luck that [may have] occurred on only a very few planets."

"As the continents arose from the oceans, minerals in the exposed rocks reacted with carbon dioxide and created inorganic limestone. At the same time, many microscopic photosynthetic organisms have carbonate shells. ...they settle to the bottom of the sea, causing a buildup that eventually becomes organic limestone."

"There has always been enough carbon dioxide to keep the Earth warm enough for life, but not too much to bake it to death."

"You may be nothing more than a complex set of chemical reactions. Solar energy raises the electrons of atoms to high levels of energy in photosynthesis, and all other metabolic reactions allow the energy to flow to background levels."

"Most enzymes are lumpy proteins that have little grooves in them into which molecules can slip and react."

"Enzymes are biological catalysts. ...Cats are also catalysts. If you wait long enough, everything in your house will eventually end up on the floor. Cats merely make them end up on the floor more quickly. ...Enzymes facilitate the natural tendency toward molecular breakdown, but they also defy this natural tendency, as long as the necessary energy and instructions are provided."

"The instructions for making... enzymes comes from DNA molecules. The DNA that encodes the instructions for one kind of enzyme, or a related set of enzymes, is called a gene."

"When one DNA molecule becomes two, the new strands are almost identical... Occasional mistakes occur during the copying process. These mistakes are called mutations. ...Most mutations... are small and have little or no significance on the organism."

"Nature selects the superior mutations. ...That is all evolution is. And there is no way in the universe to stop it, so long as no catastrophe wipes out the evolving population."

"Scientists... have used selection to get populations of bacteria to evolve the ability to use different kinds of sugar... or to tolerate different temperatures, in just a few weeks or months."

"It takes only a few years for populations of bacteria to evolve resistance to any particular kind of antibiotic..."

"Germs and disease vectors are moving targets, and evolution is the reason for this. Evolution happens in hospitals... over the course of just a few months. What you don't know about evolution can kill you."

"Directional selection causes the average population trait to change... Stabilizing selection occurs when the average population trait remains unchanged. ...This pattern has been called stasis, or equilibrium."

"When conditions change, directional selection gears up to make the population change along with it. ...sometimes environmental conditions change and then do not change back. ...Under these circumstances, directional selection has permanent and rapid effects. ...Such bursts of evolution have been called punctuations."

"There never was a Garden of Eden, but there was, perhaps, a Garden of Ediacara... blob-like creatures that lived in the sea..."

"Squirrels are not very good predators. But if a nice dinner of meat is presented to them, who are they to turn it down. ...[a] colleague, an ornithologist, captures birds in mist nets... He discovered that a cow had been walking along the nets and biting the heads off the birds. ...another colleague ...told me that it is necessary to use two layers of chicken wire ...to protect chicks from being eaten by deer. Nonhuman vegetarians, like most human vegetarians, are tempted by meat."

"Scientists today understand that there is no single, upward staircase of evolutionary progress. All species are equally evolved."

"In some ways, bacteria can be seen as the most successful life-form on the planet. There are more bacteria, and more kinds of bacteria, than any other group of organisms. They live in a greater range of conditions and have persisted for three and a half billion years."

"Natural selection capitalizes on whatever works, whether it is of greater complexity or greater simplicity..."

"Evolution has no guiding mind or innate direction. It has simply made use of new opportunities: complexity, symbiosis, sex, and altruism."

"This is why evolution appears to have a direction: there are many more ways to become complex than to become simpler."

"One of the most beautiful things about an adaptation is that once it has come into existence, it can be used for something very different than its original function."

"Almost as soon as the Earth had oceans, there were simple cells, similar to the ones we call bacteria and archaea."

"Three and a half billion years ago, Earth's surface consisted of air, ocean, rocks, and bacteria. If you went back in a time machine and got out, you would have fried to a crisp a few minutes after you suffocated."

"The bacterial world persisted, gradually building up an oxygenated atmosphere and making carbon dioxide into organic materials, for a couple of billion years. Infant Gaia had a bacterial body."

"The colonial growth form allowed a new step of evolutionary innovation. The different cells in the colony could start specializing on different jobs."

"In an evolutionary instant about 540 million years ago, there was an explosive diversification of multicellular animals. It is called the Cambrian explosion... Most of the major groups of marine animals, except fishes, appeared at that time, including sponges, echinoderms, and, most notably, arthropods."

"Bones are a way of storing calcium, which can be scarce in fresh water or in an animal's diet. If your heart muscles need calcium, they rob it from your bones. The evolutionary diversification of bony fishes might have been made possible because bones allowed them nutritional flexibility, not just flexibility of motion."

"To live on land, organisms needed vastly greater amounts of complexity."

"External skeletons are not strong enough to support the weight of large animals, and to this day terrestrial arthropods are small."

"Plants needed a new kind of support structure if they were to survive on land. ...The evolutionary innovation that allowed the first land plants to grow up into the air was xylem."

"It is the evaporation of water from plants that keeps them from overheating in the sunshine. ...Their evolutionary innovation was that the epidermis cells, with their waxy cuticles, contained stomata..."

"Insects and spiders had already been walking on land for millions of years before the first fish dragged itself out of the water during the Devonian period..."

"Insects were the first animals to fly, evolving the ability to do so at least 150 million years before reptiles and birds."

"A rich oxygen atmosphere would be much denser than today's... dense enough that delicate wings could... support the weight of a huge [wingspan a little over two feet] dragonfly."

"Most of the vegetation on the surface of the Earth had died during the Permian extinction."

"For the first three million... years, during the australopithecine era of human evolution, human brains were no larger than those of chimpanzees. That is, for 60 percent of human evolutionary history, intelligence was not a particulalarly important factor."

"The evolutionary innovation that set the human lineage apart from other mammals, from the very beginning, was bipedalism. Humans have been walking on two legs for over five million years..."

"Anthropologist Richard Wrangham points out that cooking either meat or vegetables makes them easier to eat and increases their nutritional value. Once humans began to cook their food, more nutrients were available for the continued evolutionary increase in brain size."

"Ancient bacteria had almost all the kinds of metabolism that we find on Earth today."

"Symbiosis transformed the Earth from a broth of simple cells into everything it is today."

"Billions of years ago, bacteria with two different kinds of photosynthesis fused together and formed a complex and efficient type of photosynthesis. The result was the cyanobacteria, and they were so successful that they became—and still are—the most abundant cells on Earth."

"The meager food web of the Earth became a vibrant one, based on the sugar produced by cyanobacteria."

"The cyanobacteria removed much of the carbon dioxide from the atmosphere and replaced it with oxygen. ...Because oxygen is so reactive, it must be continually renewed; otherwise it would react with everything else and be used up within a few thousand years. ...this is what happened on the young Earth—the oxygen reacted with iron in the oceans until nearly all the iron turned red..."

"The formation of an oxygenated atmosphere... shielded the planet from ultraviolet radiation, which can damage... cells. Ultraviolet radiation fuses oxygen atoms into ozone molecules, which then block ultraviolet radiation."