* Humans have proven an undeniably successful species, extending their range all over the Earth, indeed reshaping the planet to a degree. How humans came to be, remains an interesting question; it is also interesting to consider their impact on the evolution of life around them.
* The Cenozoic Era, from the planetary perspective, seems uneventful in hindsight, other than the gradual drift of the continents into their modern positions. It wasn't really that uneventful: there were three mass extinctions in the age of mammals, some 56, 37, and 34 million years ago. They were not so dramatic as the event that ended the age of dinosaurs, much less the Permian extinction, and their causes are not known. Twenty million years ago, the Earth entered into another era of climate instability, with at least eight ice ages occurring in that interval.
The later ice ages coincided with the global migration of humanity. Only a 100,000 years ago, humans were essentially just another animal species; no alien visitors to the planet would have found them much more significant than other large mammal species, except in potential. 10,000 years ago, on close inspection, the visitors would have found the human presence much more obvious, with humans found on all around the planet.
The visiting aliens still wouldn't have noticed much different in the appearance of the Earth from orbit, but humans were nonetheless having an effect on the planet. In the wake of the spread of humans, large animal species had an inclination to die out: after humans arrived in the New World, the big mammal species -- mammoths, ground sloths, giant armadillos -- disappeared.
There are questions over how much humans had to do with that disappearance, they might have disappeared anyway, but the history of human settlement does show that humans did have a tendency to kill off vulnerable large animal species. They were relatively easy prey for hominins armed with effective weapons; once the big animals got scarce, humans turned to smaller species -- rabbits and smaller antelopes -- that bred fast enough and in numbers large enough to make them harder to kill off completely.
The romantic notion that primitive societies lived in a tidy "balance of nature" doesn't get much respect these days. Richard Dawkins likes to lampoon the popular idea of the "balance of nature" as a Panglossian read on a rougher reality, pointing out that in evolutionary terms nature is competitive, with different species evolving to get the upper hand. When there's a balance, it's more like a "balance of power" than natural harmony, since the system has tendencies toward instability. Upset the balance of power, extinctions result. The bottom line is that the Earth had never seen another species like humans before, capable of harnessing sophisticated technologies, and almost from the outset humans upset the status quo.
100 years ago, the alien visitors would have found humans altering the environment of the planet, in particular leaping over the ocean barriers between continents, leading to a massive cross-infusion of foreign species into new environments, with unpredictable effects. International communications in particular allowed the spread of diseases, afflicting not only humans but other organisms, around the globe.
Today, the human imprint on the planet would be obvious from orbit without detail inspection, humans living in sprawling megacities and converting huge tracts of land to agricultural production. The spread of humans has crowded other organisms, in some cases to extinction, though some have benefited. While large predators, free-ranging herbivores, and organisms dependent on specialized environments have found the presence of humans troublesome, in the United States deer and coyotes have found the fringes of urban areas very much to their liking, and their populations have exploded.
The American opossum, once generally confined to the US Southeast, now ranges along most of the US West Coast, thanks to human transplantation. The monk parakeet, kept by humans as a pet, has established feral populations in locations as far spread as Israel and Japan. Since it is relatively tolerant of cold weather, flocks are even found in American cities as far north as Chicago and New York City. As organisms find new homes in human-dominated environments, they gradually evolve and become better adapted to those environments.
Beyond the effects on species, human activities are increasingly believed to be promoting a climatic shift. Given the global impact of human activities, a Dutch atmospheric chemist named Paul Crutzen (1933:2021) suggested that humans could be thought of as having themselves initiated a new geological age of the Earth, the "Anthropocene". Just how drastic this change in environment is likely to be remains to be seen.
BACK_TO_TOP* The rise of humans led to significant interactions between humans and other organisms, both with organisms suffering from competition with humans, as well as striking up "partnerships" with them through the domestication of plants and animals.
As the history of domestication of plants demonstrates, such partnerships were not necessarily things that humans planned out. Cultivating crop plants can be a very elaborate process, not something that human hunter-gatherers would have simply dreamed up and implemented -- and though many of our crop plants began life as perfectly edible plants, some of the wild precursors of crop plants are toxic. Eating a handful of wild almonds can be fatal.
It is apparent that the early domestication of plants was far more dependent on lucky accidents and opportunism than it was on deliberate planning, demonstrating that there isn't a strict line between artificial and natural selection. Over the millennia, farmers did in fact become more deliberate in their exploitation of wild plants, growing new crops that took more elaborate tending. The process began about 10,000 years ago, with the domestication of plants like wheat that were relatively easy to tame. Peas followed soon after, but more challenging plants like olives didn't become crops until about 6,000 years ago. Many plants we take for granted on our tables were only domesticated comparatively recently -- strawberries weren't domesticated until the Middle Ages, pecans not until the 19th century.
In this process, many of the crop plants we grow have changed almost out of recognition of their wild ancestors. Supermarket apples are three times bigger in diameter than their wild ancestors; as discussed earlier, wild bananas are small and full of seeds, supermarket bananas are big and seedless. Modern corn is a mutant monster compared to the wild grasses that it was derived from. How did these crops become domesticated in the first place, and why have they undergone such changes in form?
* One of the initial factors in the human domestication of plants was the fact that some plants effectively encouraged it. Plants don't have much ability to get around, so to reproduce, many species have enlisted the help of animals. The plants generate fruits of various sorts containing seeds; an animal eats the fruit, to then go on its way. The seeds in the fruit eventually make their way through the animal's digestive tract, to be dispersed in the animal's droppings. As strawberries grow they are green and sour; once they mature, they become tasty and advertise the fact by turning red to attract animals, who will eat the fruits and spread the seeds.
The red fruits attracted humans who ate the fruits and passed on the seeds. As humans became more settled, they set up communities with latrines, where the seeds were deposited, to then grow up and bear fruit. The first farms were likely latrines; over time, humans roughly figured out the pattern and began to deliberately raise plants for consumption. The selection of domesticated plants with larger fruits than their wild ancestors was inherent in the process from the outset, since humans preferentially picked from wild plants with the largest fruit. It wasn't part of any plan; it was just the natural way things worked.
Of course, larger fruits weren't the only selection criteria employed by ancient humans. They were also interested in fleshy or seedless fruits, oily seeds, and long fibers:
Not at all incidentally, farmers were raising crops for cloth from early on, with flax domesticated by 7000 BCE. Europe remained heavily dependent on linen, the cloth made from flax, until it was supplanted by cotton during the Industrial Revolution.
* Along with obviously desireable properties like taste and size of fruit, less apparent properties also led to the domestication of plants. For example, consider peas. In wild peas, the pod bursts open to scatter its seeds once they ripen; a single-gene mutation sometimes arises that prevents the pods from bursting. That would be effectively sterilize the plant -- except for the fact that humans harvested the pods that didn't burst, ultimately cultivating the mutant strain. Similar nonpopping mutants were domesticated from lentils, flax, and poppies.
Instead of growing in a pod, wild wheat and barley grasses feature seed heads grown at the top of a stalk that spontaneously shatter when the seeds are mature, distributing them. A single-gene mutation will prevent the head from shattering -- which again would reduce the plant to sterility, but at the same time would allow humans to harvest it. Without the mutation, grain plants wouldn't have been domesticated.
A particularly devious issue with domesticated organisms is getting them to breed true: just because we obtain useful mutant species doesn't mean the next generation will have the same mutant features. The mutants may end up being cross-pollinated with normal plants, with the next generation losing the mutant features. One way out of this difficulty is to crossbreed a plant with itself, but that's impossible if the plant species has two distinct sexes, and difficult if a plant species is hermaphroditic, but has mechanisms to prevent self-fertilization or "selfing".
Mutations arose that early farmers were able to exploit to help resolve these difficulties. Some mutants developed the ability to bear fruit without being pollinated -- for example, seedless bananas, grapes, oranges, and pineapples. Some mutant hermaphrodites lost their mechanisms to prevent selfing -- for example, plums, peaches, apples, apricots, and cherries. Some mutant grapes that normally would have had two distinct sexes ended up becoming hermaphrodites that could support selfing. Now the farmers had plants they could grow from generation to generation, obtaining the same yields, though of course at the penalty of becoming much more vulnerable to pathogens.
Through processes of inadvertent or deliberate selection for desireable traits, farmers developed farm crops of almost bewildering diversity. Some plants, like sunflowers, were bred for big seeds, while bananas were bred for negligible seeds. Lettuce was bred for leaves at the expense of seeds or fruit; wheat and sunflowers, for seeds at the expense of leaves; and squashes for fruit at the expense of leaves. Some plants ended up being bred in several different directions, for example beets, which were originally grown for leaves, still true for beet varieties known as "chard"; then for edible roots; then, from the 18th century, for sugar content, resulting in sugar beets. Cabbage demonstrates an even greater diversification, being possibly originally grown for their oily seeds, to then be bred for leaves, as with modern cabbage and kale; stems, for kohlrabi; buds, for brussels sprouts; and flower shoots, for cauliflower and broccoli.
* There were similarities as well as differences in the patterns of domestication of plants between different regions of the world, partly due to the availability of wild plant "feedstocks"; partly due to climate; and partly due to culture.
The most historically prominent region in terms of plant domestication was the "Fertile Crescent" of the Middle East. The region got a jump start because it was blessed with wild edible plant types that were easily domesticated -- wheat, barley, and peas, which were being farmed there 10,000 years ago, with flax for cloth following a few millennia later.
Cultivation of such crop plants let to the rise of fixed communities and farming practices, leading to a "second wave" of plant domestications about 4,000 years ago, the new domesticates including olives, figs, pomegranates, and grapes. These second-generation plants required more effort to farm, since they don't yield edible fruit for several years after being planted, demanding farmers who were able to plan over the long term. However, these plants were still not all that troublesome to raise if one was patient, since they could be grown from cuttings or even seeds, and they bred true.
That was not the case for the "third wave" of domesticates, such as apples, pears, plums, and cherries; they couldn't be grown from cuttings, and they didn't breed true. The only way to replicate them was via graftings onto other plants, a tricky process, and such plants were only tamed over the last few millennia. However, while farmers became increasingly deliberate in their domestications of plants, inadvertent domestications continued, with plants that were originally seen as weeds and nuisances for farming of real crops becoming crops in their own right. Examples of crop plants that started their domestic career as weeds include rye and oats; turnips and radishes; beets and leeks; and radishes.
* Roughly the same sequence took place elsewhere: early domestication of grains along with "pulses" like peas or beans, plus domestication of plants as a cloth feedstock; then raising of fruits that took some years to mature; and finally adoption of plants that were difficult to cultivate, requiring grafts or other advanced farming techniques. By Roman times, different regions had obtained their own distinctive sets of crops. Now-familiar crops from China included millet, rice, soybean, hemp, and muskmelon; Africa provided sorghum, yams, cotton, watermelon, and the famously distinctive African bottle gourd; the Americas provided corn, lima beans, peanuts, cotton (as noted, domesticated independently), cassava / manioc, sweet potato, potato, and squashes; and New Guinea provided sugar cane.
Of course, along with the differences in crops there were differences in production and consumption. In the Old World, the predominant form of agriculture was "industrialized" to an extent, with farmers planting "monocultures", relatively large plots all dedicated to a single crop like wheat or rice, using animals like horses or oxen to pull comparatively sophisticated tools like plows. In the New World, the predominant form of agriculture focused on plots of mixed crops, cultivated by hand tools; in effect, vegetable gardens. In Europe and China the predominant staples were grains, that is wheat and rice. Elsewhere, they tended to be tubers -- cassava, sweet potatoes, and potatoes in the Americas, while yams were the staple in Africa. Some regions used fruits as staples: bananas and breadfruit in Southeast Asia and New Guinea.
From Roman times, additional plants have been domesticated, but few if any of these newcomers have a level of importance that rivals that of the "classic" staples like wheat, rice, and corn. Recent crops tend to be characterized by berries -- strawberries, raspberries, blueberries, and cranberries -- and nuts -- macadamias, pecans, and cashews. There is the interesting question of why it took so long to domesticate these crops.
Actually, the Romans did grow wild strawberries in their gardens, and it seems likely they were probably grown elsewhere at times. The only problem is that producing a domesticated strawberry really worth farming was troublesome, thanks to birds. It wasn't just that the birds ate the strawberries, though that was a problem; it was that as they were doing so, they were spreading seeds from other, generally wild, strawberry plants in their droppings, and so it was difficult to maintain a separate domestic strain of plants -- wild strawberries they remained. It wasn't until nets and greenhouses were introduced that humans were able to defeat the birds and develop giant domestic strains of strawberries.
In the end, humans have only domesticated a tiny fraction of the wild species of plants -- dozens as opposed to hundreds of thousands -- with the limited diversity of crop plants making them unfortunately vulnerable to pathogens. In the beginning, the domestication process was inadvertent, almost unconscious, a combination of opportunistic accident and genetic accident. Darwin began his ORIGIN OF SPECIES with a discussion of artificial selection of domesticates as a means of laying the groundwork for a discussion of the natural selection of wild species. Although critics of Darwin's work make much of the difference between artificial and natural selection, he understood that the dividing line between the two is very faint.
While modern crop plants have in some cases been altered almost out of recognition to their wild ancestors, as discussed previously humans did not create the mutations that produced those changed forms -- they simply exploited natural mutations, ones that in many cases would have been fatal to the wild plant ancestors. In the 21st century, we have an increasing capability to basically redesign organisms, with the potential for a revolution in food production. One very active field of investigation at the present time is the use of genetically-modified yeasts to produce flavorings, food colorings, odors, even drugs and silks, with advocates believing efforts to that end are very likely to pay off quickly.
BACK_TO_TOP* The issues involved in the domestication of animals have some similarities and some differences with those involved with the domestication of plants. As with domestication of plants, the story of the domestication of animals has a component of opportunism and inadvertence.
As noted, some wild animals have adjusted to the human presence better than others; a few have acquired partnerships of a sort with humans. This is a tricky matter, dependent on a number of considerations:
The dog is one of the oldest and most flexible of domesticated animals; all dogs are derived from the wolf, the general belief being that wolves became "camp followers" of human groups, scavenging from their refuse, and gradually became more integrated with humans. Humans were keeping dogs well over 10,000 years ago.
Dogs, though basically carnivores, are not all that fussy about what they eat, they mature quickly, they breed easily, and as a rule there is no animal that is fonder of human company -- partly thanks to the fact that they are a pack animal and will submit to a pack leader. They have acquired a wild range of variation in form under human selective breeding, and no animal has served a wider range of purposes, including hunting, herding, guarding, assistant to the blind, and even beast of burden -- sled dogs are able to haul loads in snow that would bog down most other domesticated animals used for transport. They are also used to an extent for meat and fur, but there are generally better sources for both, and the inclination of many cultures to see dogs as "part of the family" tends to work against their use as "livestock".
The domestication of the dog was followed several thousand years later by the domestication of ungulate livestock: sheep, goats, pigs, and cattle -- and since that time, have been followed by other familiar farm animals: chickens, donkeys, ducks, horses, geese, turkeys, and honeybees. Cats were domesticated as well, though they are not always seen as particularly so: as human agriculture spread, so did the house mouse, and cats began to hang around human settlements to prey on them and other vermin. It is said with some truth that cats more adopted us than we adopted them. There were also domestications of other animals that remained more regional or specialized: water buffalos, camels, reindeer, yaks, llamas, alpacas, and silkworms.
The modifications of animals under domestication vary over a wide range. Cats are typically not all that genetically distinct from their Middle Eastern wildcat ancestors, the difference being characterized as comparable to or even less than, say, the typical difference between Spaniards and Swedes. Many breeds of dogs of course have been massively altered from the wolf ancestors. Genetic analysis has shown that the wide variations in dog forms are actually due to a fairly small number of genetic changes, with research providing insights to the effects of these genes.
While creationists argue that the changes humans have exploited via selective breeding in dogs are "degenerate", and it might seem so from examples such as pugs or pekinese, some dog varieties have impressive specializations -- greyhounds of course are optimized for speed, and some breeds are startlingly intelligent. Anybody who has ever watched a blind person under the "care" of a seeing-eye dog can hardly help but be impressed by the dog's dedicated sensibility, or think of how unlikely it would be for the dog's wolf ancestor to perform such a feat. It might have been intelligent enough, but it would not have been temperamentally capable of doing the job.
Silkworms are another interesting example of modification under domestication, with silk moths easily handled by humans and even unable to fly. Some domesticated animals raised largely for ornamental purposes have acquired wild variations under domestication -- pigeons have already been mentioned, and goldfish can be seen as even more extreme, with exotic goldfish exhibiting such bizarre features as huge bubble eyes, topknots, and overgrown or duplicated tailfins.
Compared to the number of potentially useful animals around the world, the number that have been domesticated is, as with plants, actually pretty small. Of course, most animals can be "kept" in zoos, though some more easily than others; some animals not normally regarded as domesticated can be put to work given the perceived need -- there are naval forces that train seals and dolphins to perform harbor security, but outside of such specialized roles, few could see any reason to go to the expense of tending to such creatures. Indeed, the push is to replace them with submarine robots that are far easier to transport and can be put in storage when not needed. Kept animals can also be raised commercially in some cases. For example, the American bison is raised in enclosed ranges in places in the US West to provide a specialty meat, but that hardly makes it a domesticated animal -- it's much too bad-tempered and dangerous. Nobody in their right mind would approach one the way we would, say, a cow.
In addition, there are animals that obviously could be domesticated if there were the will to do so. Zebras can be broken to harness and even ridden, but as a rule it's difficult to do; they usually resist stubbornly and viciously. Some races of zebras seem to be more docile than others and they could certainly be bred for greater docility, but why bother? There's nothing a zebra could do that we haven't long had a range of horses that could do better, from ponies for children to big Percheron plow-horses, and so there's no strong incentive to domesticate the zebra. All it would amount to is a stunt, and a lot of work for just a stunt.
* Again, the introduction of genetic engineering clearly changes the rules for domestication of animals, opening up the possibility of, say, creating dogs with near-human levels of intelligence. Such ideas are fascinating, but they are also a little unsettling. Genetic engineering of plants is one thing that has proven politically troublesome enough; the idea of coming up with sentient "neodogs" is downright scary. Would such creatures have the rights of humans? Would killing one be murder? Since we don't have an immediate capability to produce sentient dogs, the question is academic for the moment, but it may not always remain so.
BACK_TO_TOP* While the question of abiogenesis, the origins of life, was sidestepped earlier, it is a field of energetic research. Creationists like to insist that it is impossible for life to have arisen from nonlife, pointing to the complexities of even simple bacterial cells, then performing calculations based on those complexities that show the probabilities of life spontaneously arising from nonlife are vanishingly small -- comparing it to the idea that a jumbo jetliner could be thrown together by a tornado running through a junkyard.
However, that's absurd; if life didn't arise from nonlife, we wouldn't be here, and so the only argument is how it happened, with "magically poofed into existence" being the laziest and least credible proposal. As far as the probability calculations go, they're silly. Texts on probability often use, say, playing cards as examples, deriving the probability of obtaining certain hands of cards. That's fine in itself, but with playing cards, we have an effectively complete knowledge of the scenario, knowing the numbers and types of cards, and the rankings of the various hands. In the case of the origins of life, we have nothing close to a complete understanding of the scenario -- that's why the research is being done -- and so there's no realistic way to evaluate the probabilities.
A simple calculation of the probabilities of the synthesis of a molecular system on the basis of its arrangement is meaningless if we don't really know how it was formed. After all, on the basis of its arrangement, the probability of the formation of a salt crystal ten atoms on a side is no less than one in (2^1000)/24 = 4.17E300. Of course, such a tiny salt crystal is perfectly ordinary, its formation not being dependent on "random" processes.
The "tornado in a junkyard" comparison is based on a blank misunderstanding of abiogenesis studies. Nobody working in the field believes that even a bacterial cell could have spontaneously assembled. All research from the earliest days of the field has assumed that life began with some simpler system that gradually became more elaborate – that is, originating with a "LIFE/0", evolving through "LIFE/1" and then "LIFE/2" and so on, to ultimately become "LIFE/NOW". Creationists insist the odds of the emergence of even a simple self-replicating molecular system are vanishingly small, but given high rates of chemical reactions on a young and geologically-active Earth, working as a planetary-scale "bioreactor", over hundreds of millions of years, the emergence of life might well have been a certainty.
Until we learn more, all we can say about the probability of the spontaneous emergence of life on Earth is that it's greater than ZERO and less than ONE. Life may indeed be a freak accident of the cosmos; the Earth could be the only planet in the entire Universe where it has arisen. Few would find that idea any less than staggering: there are 100,000,000,000 stars in our own Galaxy, and even if life is a one in a million thing, that means 100,000 star systems where life has arisen, in forms that could resemble life on Earth, or could be radically different from it. We are simply ignorant of how widespread life actually is, and ignorance gives us no basis for useful argument about anything.
* The first serious work on abiogenesis was conducted in the 1920s, when the British biologist J.B.S. Haldane (1892:1964) and the Soviet scientist Alexander Oparin (1894:1980) proposed relatively detailed models for how life might have arisen. Haldane postulated the rise of biomolecules in a warm body of water saturated with simple biomolecules, which Haldane called the "primordial soup". Oparin started out with a similar environment, but suggested that oily blobs could have led to the creation of simple cellular structures.
The "primordial soup" notion is now seen as misleading, since nothing much is likely to happen in an unenergetic broth of chemicals. In modern times the focus has shifted to ocean-floor volcanic vents, which are not only chemically diverse and energetic environments, but also support the growth of "chimneys" above the vents -- tall porous mineral structures that provide excellent substrates for chemical synthesis. Traditionalists had problems with the volcanic-vent scenario when it was introduced, saying it was too harsh an environment for life to arise; but there is clearly a gradient between the vent and the open sea, giving a range of conditions. There was a "Goldilocks" region that wasn't too harsh and wasn't too calm. There's plenty of life around volcanic vents in the current day, and the concept is now generally accepted as the most interesting scenario.
In any case, nobody paid too much attention to the ideas of Haldane and Oparin, mostly because the era's knowledge of biochemistry was so primitive. However, an American chemist named Harold C. Urey (1893:1981) was impressed, and eventually decided to perform a simple test of the matter. In 1953, he set a graduate researcher named Stanley Miller (1930:2007) to the task, with Miller recirculating what Urey believed to be a sample of the primordial atmosphere -- consisting of hydrogen, ammonia, and methane, but no oxygen, or what became known as a "reducing" atmosphere -- in a flask that was subjected to electric sparks. After a few weeks of this treatment, organic materials emerged, notably many of the amino acids used to build up proteins.
The Miller-Urey experiment was trumpeted up in the textbooks as a fundamental breakthrough on abiogenesis, but it was nothing so dramatic. Hydrogen is a light gas and tends to dissipate into space from the atmospheres of relatively small planets with low gravity like the Earth, and most geoscientists believe today that the primordial atmosphere was "nonreducing", composed of carbon dioxide and some nitrogen, and at least initially experiments that tried to duplicate the Miller-Urey experiment with such an atmosphere were duds.
The production of amino acids was also not so dramatic. They're easily generated, actually being found in interstellar gas clouds; indeed, in the early history of the Earth, biochemical building blocks like amino acids were continually falling on the Earth, brought in by comet impact. The chemical reactions that generate such building blocks are exothermic -- they release energy -- and so they're thermodynamically straightforward. The bad news is that assembling amino acids into proteins is an endothermic reaction, meaning it requires energy. That doesn't rule out the self-assembly of proteins, but it makes it more troublesome to consider.
In addition, life can be divided into two primary components: replication based on DNA and RNA, and metabolism based on proteins, with the two sets of processes strongly interlinked, neither being particularly useful without the help of the other. The Miller-Urey experiment focused on the synthesis of protein components and said nothing about the synthesis of DNA and RNA. Early experiments to simulate the spontaneous origin of RNA proved frustrating.
Although the Miller-Urey experiment has been heavily criticized, it does have its place of honor in the history of science as the first serious experiment to investigate abiogenesis. It has been followed by improved variants operating on a range of scenarios, and recent experiments have been encouraging.
It turns out that a reducing atmosphere does a perfectly good job of producing amino acids, as long as reactive nitrogen compounds formed by the sparks are soaked up by trace "buffering" chemicals. Similarly, difficulties in the synthesis of nucleic acids increasingly seem to have been rooted in our past failure to understand that there are many different ways in which nucleic acids can be synthesized; experiments based on revised assumptions are now showing it's not as troublesome as it was once thought to be. There was clearly no shortage of essential biochemical building blocks on the early Earth, such as amino acids; the nucleotide bases used to build up DNA and RNA; lipids, the basic molecules of fats; and carbohydrates, or chains of sugars.
* New theoretical models have been developed in parallel with better experiments. One scheme, with its roots in the 1960s with the British-American chemist Leslie Orgel (1927:2007) of the Salk Institute in the US and promoted by German biophysicist Manfred Eigen (1927:2019) of the Max Planck Institute in Germany, postulates that in the beginning, life was based on RNA. One of the interesting things about RNA is that, unlike DNA, it does not need proteins for replication. RNA is capable of acting as an (admittedly inefficient) catalyst to support RNA replication, with such catalytic RNAs known as "ribozymes".
Biologist Jack Szostak of Harvard (born 1952), a Nobel-prize-winning biochemist and a well-known figure in the field of abiogenesis research, has pointed out that lipids will easily form up spheres or "vesicles". He has conducted experiments that show that if RNA is mixed with lipids, an envelope will grow around the RNA, forming what might well be a protocell. These experiments have been able to produce envelopes that can grow by accumulating more lipids, and allow nucleic acids to leak through.
Biochemists tinkering with RNA have also found that it easily associates with short protein segments or "peptides", forming "ribonucleoprotein complexes" that could represent the roots of the mutualistic system of replication and metabolism seen in modern organisms. It is becoming apparent the emergence of the first protocells was actually in one step -- not RNA or peptides or lipids, but all three of them together, achieving a synergy in collaboration, a kind of molecular "mutualism" or "symbiosis".
This notion was the basis of the "garbage bag" world, the best-known proponent of which is physicist Freeman Dyson (1923:2020), in which vesicles form up around agglomerations of RNA and peptides and whatever chemical junk is available in the environment. It's not remarkable that there would be a mix of things in the vesicle; it would be more unlikely that they would be sorted out. Should one of these garbage bags then be able to grow by absorbing more molecular materials from its environment, it could ultimately split, to then grow and split again. At the outset, it would be a very awkward and inefficient excuse for an organism -- but as replication continued, lineages would arise that were more efficient, leading to something that looked more orderly than a bag of garbage.
Even if the emergence of a self-replicating garbage bag is judged a "one in a trillion" event, conservatively assuming that a garbage bag is formed once a second over the entire early Earth, then on the average such a "one in a trillion" garbage bag would arise about once every 32,000 years. Raise the odds against it by a factor of a thousand, that's about 32 million years, not a long time in the Earth's history. Probability calculations reinforce abiogenesis; they don't undermine it.
Of course, there's no reason to think the emergence of protocells only happened in one place in one time; there may have been multiple, roughly parallel origins of life -- "LIFE/0A", "LIFE/0B", "LIFE/0C", and so on -- with the different LIFE/0s competing with each other, or symbiotically associating to result in "LIFE/1", acquiring complexity by mergers from separate roots.
Creationists ask why we see no evidence such protocell systems in the world today, but the answer's obvious: as each more efficient level of cell systems arose, it shoved the previous one into extinction by soaking up all resources available and devouring its predecessor. Modern organisms are found all over the planet, in surprisingly hostile environments, even deep into the Earth, and it is very difficult to set up a lab environment free of biological contamination. Any resources that could support a second abiogenesis would be quickly gobbled up by the life that exists. The only way we will likely observe any early protocell system is in a lab experiment, and since researchers don't have a planetary-scale bioreactor like the primordial Earth and hundreds of millions of years to experiment, that remains a challenge.
BACK_TO_TOP* This document began life as part of a survey of evolutionary science titled INTRODUCTION TO EVOLUTIONARY SCIENCE I released 2008. It was really three different documents glued together, so I finally decided it would be more convenient in all respects to split it down into three separate documents.
I have to apologize to readers for bringing up creationism in this document, since it's nothing but a distraction. If creationists don't want to accept evolutionary science, fine, they have a perfect legal right, nothing to discuss. However, they are not happy with simply rejecting evolutionary science, instead insisting that the sciences should, or even do, back up their position. Nothing to discuss there either; the science community overwhelmingly regards creationism as a joke, to the extent any attention is paid it at all. One might as well claim that Mexicans speak French instead of Spanish.
Creationists get particularly agitated at abiogenesis studies, claiming the subject as a "killer argument" in their favor. However, all they have is an assertion that because we don't really know how life began, that somehow proves it must have been created by magic -- or some unexplained and, it seems, permanently unexplainable process indistinguishable from magic: "We are clueless, therefore magic!" They persist in this assertion even when it's pointed out to them they are selling nothing but their own ignorance. They're not going to go away, of course; but all the sciences can do is shrug, and go on about their work.
* Sources include:
Illustration credits:
All the clade diagrams are my own effort -- they’re in an unorthodox format, but space constraints made it necessary.
* Revision history:
v1.0.0 / 01 jun 17 / Split off from INTRO TO EVOLUTIONARY SCIENCE. v2.0.0 / 01 may 19 / Added geological history. v2.1.0 / 01 may 21 / General review & update, illustrations update. v2.1.1 / 01 apr 23 / Review & polish.BACK_TO_TOP
