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Biogenesis

Timeline

       Four billion years ago, approximately a hundred million years after the emergence of solid land, life arose on Earth. We are not sure which of several hypothesized mechanisms accidentally created the first self-replicating molecule, most likely ribonucleic acid (RNA). It could have arisen from deep-ocean hydrothermal vents, from pools of organic “soup,” from alkaline seepages along the ocean floor, or from several other processes. However it happened, once this molecule was formed, it copied itself repeatedly. But not all copies were perfect, either through copying errors, or through environmental attacks on the molecules. The mini-environment in which the molecule first arose was soon filled with a number of slightly different versions of the original.

       And evolution began. The four keys to evolution, as enumerated by Charles Darwin, are variation in a population, inheritance of traits from a parent, reproduction, and limited resources. This mini-environment of self-copying molecules had all the ingredients necessary. Once the environment was filled, offspring could only be made by those who could compete better for the scarce resources, or who could survive and copy at the fringe or outside the environment. Molecules with beneficial variations survived and reproduced in greater numbers. Those without did not. And the cycle repeated: variation due to copying errors or the environment, better survival and reproduction by those with certain beneficial traits, a new generation with variation, a new wave of survival and reproduction. Individual molecules usually did not change, but the population changed generation by generation, one tiny modification at a time.

       One of the reasons it is sometimes hard to fathom how evolution works is that it is excruciatingly slow. One form does not change into another over one generation, or over a dozen. The time for the evolutionary step from the first living, replicating molecules to the first organisms that had cellular walls may have been on the order of a hundred million years. That means billions of generations of trial and error passed, tiny step by tiny step, until one-celled organisms emerged on the Earth. We can hypothesize about possible steps along the way: the accidental changes that allowed a self-replicating molecule to make proteins which attached to itself, thus either shielding it from harm or helping draw in building blocks for new copies or energy for the steps necessary to make a copy; the growth, generation by generation, of such shielding until it curved around upon itself and created a bubble; the advantages that the organism would have if the key molecular strand were on the inside of the bubble and the bubble allowed only nutrients through; the additional nutrients it would receive if the bubble folded back upon itself to increase surface area; the extra layer of protection the key strand would have if it pinched its own fold off from the main bubble in a tiny central bubble. Whatever the exact steps, by about 3.9 billion years ago single-celled organisms had developed, using deoxyribonucleic acid (DNA) as the key molecular strand, RNA as a messenger, and proteins built in the folds of the DNA and RNA as the building blocks of the other components of the cell.

       Eventually, some cells began to use forms of photosynthesis to generate energy for the cellular chemical reactions. After more evolution, cyanobacteria evolved that produced oxygen as a waste product. Again, the timeframes are enormous; we have traveled close to a billion years from the first one-celled organisms to get to cyanobacteria, which arrived around 3 billion years ago. The waste oxygen that they produced increased in the atmosphere to a much higher concentration than had ever been on the earth. Oxygen was toxic to most of the one-celled life at the time, and killed off the vast majority of it. However, some of the cells had various random mutations that allowed them to tolerate higher concentrations of oxygen, and in maybe a half a billion more years, evolved the ability to use oxygen as a way to extract energy from food.

       Over a billion years after that, around 1.2 billion years ago, some of these cells evolved a novel strategy to increase the probability of successful offspring: the ability to share DNA with another similar cell. The advantage this conferred was that beneficial mutations could be shared within a population and would evolve to the higher percentages in the population more quickly. For example, if one cell had a mutation that made it use food more efficiently, and another cell had a mutation that made it produce more offspring, some of the offspring of the shared DNA between these two might have both mutations, and would thus have a much greater advantage over its peers. This advent of what we call sexual reproduction greatly sped up the evolutionary process.

       Another 200 million years later or so, the first multi-cellular organisms arrived on the scene. For organisms, the ability to sense and move toward food nearby spurred the evolution of nerves, sensory organs and eventually a brain. Such organisms are called animals, and the first rudimentary animals in the ocean, such as jellyfish and sponges, arrived about 600 million years ago. Animals continued to achieve evolutionary success through a variety of means. Some became predators eating other animals and gained food through size or speed or enhanced sensory ability. Some ate only plants and succeeded where there was an abundance of plants available to eat. Some reproduced in such large numbers that even though most of their offspring might starve or be eaten, some would survive. Some evolved the ability to eat many different plants and animals, ensuring that food would always be available. Some developed armor-like structures or speed to either defend against or run away from predators. Most animals evolved based upon a combination of these various strategies for reproductive success. This great explosion of evolutionary changes and strategies among animals occurred relatively quickly on the evolutionary timeline, and animals soon filled the oceans. This event is called the Cambrian explosion, and occurred roughly from 565 to 525 million years ago.

       In the next 200 or so million years, plants and animals finally evolve to colonize the land. The animals included insects, arachnids, and some that resembled amphibians and reptiles. The Earth was now teeming with life. But about 250 million years ago, there was a massive die-off, killing maybe 95% of all animal species. Scientists are still debating possible causes of this mass extinction (called the Permian-Triassic extinction event, or P-T event), and have several different theories that alone or in combination could have caused it. However it happened, it set the stage for the emergence of mammals and birds from early reptilian ancestors. Also appearing on Earth were the first giant reptiles, the dinosaurs. In the 185 million years following the P-T event, plants evolved flowers, birds evolved flight, and mammals evolved placentas. Mouse-sized placental mammals from that time are the ancestors of all placental mammals on Earth today, including humans. Man’s last common ancestor with rodents was about 100 million years ago.

       But about 65 and a half million years ago, a meteor struck the Earth at what is now the Yucatan peninsula in Mexico, and the ensuing environmental catastrophe wiped out about half the animal species. This event is known as the Cretaceous-Tertiary extinction event (the K-T event), and the change in temperature wiped out virtually all of the cold-blooded dinosaurs. Without the threat of large predators, mammals diversified, and larger mammals appeared. Many species of mammals, with a wide range of adaptations, arrived. The world had begun to take on a familiar appearance, with a high degree of variety in flora and fauna, and mammals dominating the land.

Theory & Evidence

       The process of evolution starts with the first self-copying molecule or process. What caused this first molecule or process to arise is still unknown, although we have several hypotheses about how this happened. But evolution is only concerned with what happened after that molecule came to be. Once it did arise, the process of evolution took over. Charles Darwin described evolution as being based on four principles: variation between individuals, the inheritance of traits from a parent, the ability of an organism to reproduce more than just enough to replace itself, and a limitation of resources such as food or habitable space. We will describe these in a bit more detail.

       Variation between individuals arose even before there existed sexuality and sharing of genetic material (which greatly speeded up the process of evolution). Since organisms were merely copying themselves, it stands to reason that some of these copies were not perfect copies. These imperfections arose due to both environmental factors such as radiation or contamination, and some inherent uncertainty in the copying process. Any evolution toward perfect copying would be limited by the loss of adaptability of the resulting offspring. If you only make perfect copies, and the environment changes so you don’t have enough resources to survive, all of your offspring will die, too. But if you made some mistakes, some of your offspring might be able to adapt to the new environment.

       There is a common misconception about evolution that species had members that changed, morphing from one type of animal to another. Such thinking is related to our human experience, where we see events occurring over days, years, or decades. Evolution is much, much more subtle than that. What evolution does is take those very minor copying imperfections, the variations we already described, and pass them on to the next generation. Every subsequent generation will inherit the new genetic sequence, whether it helps or hurts them. When the new generation is confronted with new challenges for survival, some will be better suited to live and reproduce than others. Those who survive and reproduce at a greater rate will have more offspring in their next generation. No individual changed any more than the initial copying error. But the group of organisms did change, because each new organism inherits changes from the previous generation. Such changes are quite small, but they add up over long enough periods of time.

       So we understand variation between individuals, and the inheritance of traits from parents. Now we still need sufficient reproduction to grow the group, or at the very minimum keep it stable. If a copying imperfection lowers the rate of reproduction (net of other mortality factors) of an organism, it will lose out to those organisms that did not have such an error, over enough generations of low reproduction.

       The final piece of Darwin’s theory is the limitation of resources. Even if a species were to end up, for example, on an island where it had more than enough food, water, and space to live, it would soon reproduce to the level where there was competition for food, water, or space. Resources are always limited or on their way to being limited. With such limitations, mutation through variation, reproduction, and inheritance can give a competitive advantage to particular imperfections. In organisms that share genetic material, the new successful genes will spread throughout the population over generations of reproduction. Simultaneously less successful genes will dwindle in numbers, swamped by the more successful ones.

       Those are the basic ideas behind Darwin’s theory. He also coined the phrase “survival of the fittest.” Again, this may cause some confusion, due to the lens of human experience. Say we compare a handsome, strapping man married to a beautiful, athletic wife. Both are intelligent and charming, and together they run a successful business. They have a handsome son and a beautiful daughter, both very healthy, who they dote upon. Compare them to another couple, both of whom work at low-wage jobs. They are out of shape, unintelligent, have a horrible diet, and have a rocky marriage. They have seven children, all of whom are dirty and ill-mannered, and two of whom have diseases due to poor pre-natal care that will not allow them to reproduce. Who was the fittest? From an evolutionary point of view, it was the second couple, because they succeeded in reproducing more children capable of reproduction than the first.

       So the theory of Evolution, basically tells us that all life (including humans!) evolved from earlier life forms through successive generations of mutation, combination, and selection. It is the rule of “survival of the fittest,” and it explains how humans and all life on earth came to be. Below we will list some of the primary pieces of evidence for evolution, and direct interested readers to the Book of Books for further reading on evolutionary theory.

Common Physical Structures

       We can find in diverse living animals structures that are shared, and almost identical. For example, the formation of blood vessels is very similar in almost all mammals, from humans to hamsters to hippopotamuses. Most mammals share appendages with 5 countable “fingers” or “toes,” even if some of those digits have since evolved to fit other purposes, such as the formation of wings in bats. As you narrow down to smaller, more-closely related groups, you can find specific similarities that are not shared by larger groups. The basic cycle of converting food into energy is nearly identical for all primates, who need for vitamin C in their diets. Most mammals produce their own vitamin C, but only those mammals which look the most like humans have the same dietary requirements as humans. Our common ancestry is more recent than that with hippos or hamsters.

       And we can see what happens when diverse, distantly-related creatures evolve similar structures to fill the same purpose: the structures end up different. Humans and squid and bumblebees have all developed eyes, but the eyes are extremely different. They don’t show much in the way of common ancestry, unlike the eyes of humans, gorillas, and chimpanzees, which are extremely similar. Here’s a great quote on the topic from the late Allan Glenn:

“Without evolution, what reason is there to suppose a bat’s wing—meant for flying—will have more similarity to a hoof than any bird’s wing, also meant for flying? What reason is there to suppose shark and dolphin flippers, which function identically and can look similar enough to fool mariners, are much more different—on the inside—than a dolphin's flipper and a human hand, which have totally different ‘purposes’?

“Evolution answers this. The common ancestor of all mammals had a paw, and no matter which environment its descendants moved into, all of their paws/claws/feet/wings/flippers/hooves are obvious modifications of that original structure—muscle groups, bone and all. The same answer applies to vestiges. And this, along with a huge number of other experimental consequences, is perfectly logical and downright expected if evolution is true.”
-- Allan Glenn

Fossil Record: The Law of Fossil Succession

       A simple conclusion that can be found from a quick examination of the fossil record is stated in the Law of Fossil Selection. This states that as you dig down into older and older layers of rock, we can work our way back in time to where there are no birds, then no mammals. Further back in older layers of rock we eventually lose reptiles, then fish, then shells, and far enough back there are no animals at all. Furthermore, geologists discovered that many types of rock can be time-dated by the relative levels of radioactive elements inside the rocks at formation. With this radioactive “clock” inside rock formations, we can add times to the fossils we see, and come up with a timetable for the evolution of various animals like the history.

Developmental Similarity

       Developmental similarity is way of saying that embryos of different animals can look and be structurally very similar in the womb or egg, especially in the early stages. The very early stages of a human fetus look almost identical to the earliest stages of the fetus of a monkey, a dog, or even a chicken. In some cases, these similarities can extend beyond the obvious observation that monkeys, dogs, and chickens have heads and forelimbs and hindlimbs just like humans. They go through almost the exact same sequence of embryo formation, especially in the earliest phases, sometimes even if that step is not necessary for the final animal! For example, dolphin embryos in the womb form hind legs, just like human embryos. And then, after those legs are formed, they are reabsorbed, and the dolphin calf is born without legs. Why did those legs form in the first place? They are a developmental similarity, and they also fit in with our next topic, evolutionary leftovers.

Evolutionary Leftovers

       So if all this evolution happened, where are the transitional forms? Humans don’t have extra, useless appendages like tails, right? Wrong, actually we do have extra pieces, and we even have a tail! The human tailbone, the coccyx, is our evolutionary relic from the time when our distant ancestors had tails. Even today, children are occasionally born with a birth defect where the signal to cancel the formation of a tail has failed. We also have other evolutionary relics like our appendixes and the nipples on men. The appendix served a purpose at a time when much more of our diet consisted of insects, and nipple formation occurs in the embryo before sexual differentiation starts.

       In animals other than humans, the transitionality can be even more striking. Why exactly do elephants and manatees have toenails? They serve no purpose whatsoever, besides showing us that in the distant past their ancestors had separated digits and claws. Why do whales have legs? They don’t have legs, you say? Sure they do, deep inside whales they have leg bones. Totally useless leg bones. And then there are gannets, birds that dive into the water for food. Since they dive, they have no nostrils, so water can’t get in. But gannets do have completely developed nasal passages that are permanently sealed off. Why are the passages there in the first place?

Molecular and Genetic Evidence

       If you go down farther to the molecular level, you can find more evidence for evolution. Did you know birds have the genes for teeth? And horses have the genes for toes? In both cases, those genes are turned off by a different gene. But the evolutionary relic still exists in their genetic code. Beyond that, we can find similarities on the molecular level. Hemoglobin is used to transport oxygen by virtually all multicellular animals. We all use the same structure to process oxygen because we all came from the same ancestor.

Failure of the Organs Are Too Complex Hypothesis

       As we did in the Book of Genesis, we’ll cover one of the most common counterarguments, and show why it is false. (This hypothesis is one offered only by creationists. For a full debunking of all creationist claims, again please see the works in the Book of Books, or the list at www.talkorigins.org/indexcc/.) Creationists often say that an organ—they usually cite the eye—is too complicated to develop on its own. After all, what use is half an eye? How come we don’t see creatures with half an eye?

       Well, instead of dismissing such a thing out of hand, let’s look at the actual evidence like scientists. Eyes might form from light-sensitive patches, which would then evolve to form an indented shape and eventually a small hole to help focus the light. Do any animals have that? Let’s consider the flatworm, which has light-sensitive eye spots, exactly the first step we’re looking for. Next we look for indented eyes. The pit viper has just such organs, called titular pits, that sense infrared light, and similar hole-type eyes are found in some invertebrates. A slightly more developed eye called the “parietal eye” in lizards (it’s on the tops of their heads) has a rudimentary retina and lens, much less complicated than their actual eyes. The box jellyfish Cubozoa has rudimentary retina-lens eyes (along with pit-type eyes) that are even more developed. So we have multiple examples of transitional eye development from spots to the complex retina-eyeball-lens of most vertebrates, plus the divergent eye types of different groups, as mentioned earlier with respect to humans, squid, and bumblebees. There are plenty of “half-eyes” out there, if you take the time to look for them instead of dismissing such a thing out of hand.

       Was there enough time to develop eyes? A few scientists tackled that question, and with a little math, came up with a requirement of approximately 400,000 generations to develop a full eye such as that in mammals like humans. Since animal life has been on the earth for some 600 million years, it looks like there was plenty of time for eyes (and other organs) to evolve. The “organs are too complex to have evolved” hypothesis is discarded, under the weight of contrary evidence.

Conclusion

       There is much more evidence supporting the Theory of Evolution than just the short summary above. Again, the theory is so robust that it may be accepted as scientific “fact,” just like the Big Bang theory. If you would like more details on the evidence for the natural development of the universe and the world we live in, or if you would like information on the falsehoods and distortions of creation science, there is a section of the Book of Books listing modern works on the Big Bang and Evolution. There is a great deal of information available on evolution at EvoWiki (www.evowiki.org) and on creationist claims at the Talk.Origins website (www.talkorigins.org/indexcc/) as well. We encourage the interested reader to seek out those works.