A Brief History of Life on Earth
There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. —Charles Darwin
It is no valid objection that science as yet throws no light on the far higher problem of the essence or origin of life. Who can explain gravity? No one now objects to following out the results consequent on this unknown element of attraction. —the same guy
Building a better mousetrap merely results in smarter mice. —the same guy again
Recently I engaged in a friendly discussion of evolution from a more or less Buddhist perspective, with venerable Ajahn Punnadhammo and my good friend Brian Ruhe. Brian naturally prefers to believe that we are here not because we evolved here, but because space aliens genetically engineered us. The ajahn accepts the existence of evolution, but considers mere random mutation combined with natural selection insufficient to explain our existence, and so he favors an almost Schopenhauerian idea that mind interacting with matter helped to guide evolution. In the video I pretty much endorsed straight scientific theory on the subject, but can accept the power of karma or Will as a guiding force in evolution.
Then, just a few days after participating in the video, I watched a debate on YouTube between four scientists: two endorsing evolution and two devout Christian ones endorsing creationism and “intelligent design.” Although at least one of the pro-evolution scientists was a smug atheist and materialist, the creationists were on the defensive almost the whole time…because their argument amounts, essentially, to “We can’t explain how life evolved, especially at the cellular level, and so God must have done it.” One argument the more intelligent of the two creationists used was that there is no way to explain how bacteria evolved into eukaryotes, or cellular life with a nucleus and organelles like mitochondria—which is one thing I’ll eventually get back to here.
So evolution has been on my mind recently, and once again I am struck by the number of relatively intelligent, rational people who reject organic evolution by more or less Darwinian means. Some prefer aliens like Brian does, and others prefer to see some psychic element to it, much like the ajahn did in our video. Others, of course, prefer the traditional Christian explanation of “God did it.” Both creationists in the debate I saw went further and preferred the idea that the earth really is less than ten thousand years old.
Partly because I am bemused by the multitude of intelligent people for whom evolution is just plain unfashionable, I have decided to give a brief history of life on earth in accordance with mainstream scientific theory. It is a vast, magnificent theory in my opinion, with more objective, demonstrable evidence in favor of it than any other that I know of. All other theories, as far as I can tell, amount to appeals to ignorance: science can’t explain EVERYTHING, and therefore it is wrong. Or else they are based on religious dogmatism. Maybe the ajahn is right, though the evidence does indicate very clearly that evolution is an objective fact. The fossil record as well as vestigial organs in some animals, embryology, etc., show that life is gradually (sometimes even quickly) changing over time. Anyway, most of what follows is derived from what I learned in college when I was much younger than I am now. I have no idea what is taught in evolutionary biology classes nowadays; it may be that leftist politics has somehow thrown a wrench even into THAT field of study. I dunno. So we may as well begin with the beginning of life on this planet.
In the beginning, say four and a half billion years ago, the earth was a volcanic hellscape with asteroids occasionally smashing into that hellscape as the interplanetary debris accompanying the formation of the solar system gradually crashed into something, achieved a stable orbit, or was flung into interstellar space. This stage of the earth’s evolution is fittingly called the Hadean period. After less than a hundred million years, the earth’s surface had cooled sufficiently to allow for liquid water, although the atmosphere consisted mainly of volcanic gases like methane, ammonia, CO2, sulfuric acid vapor, and so on. Scientists have claimed that within another few million years, very primitive life had already begun here. So life began almost as soon as it was possible for organic, water-based life to exist.
Since the origin of life is one of the main issues flung out in debates to deny evolution, or at least to deny that life could have begun naturally here, I should spend some time explaining what I learned in the 1980s on the subject. I will ignore the panspermia hypothesis, mainly because it simply kicks the can down the road by saying that it is hardly likely to have begun here, so it began somewhere else. But it was hardly likely to begin anywhere, in accordance with the panspermia idea, which is the main reason why the panspermia idea was born. But the assumption that life began soon after the earth’s surface cooled down enough for liquid water is intriguing.
Scientists back in the 1970s, I think, performed a few experiments testing the possibility of life arising on the Hadean earth. They took an airtight container and put into it some water and the same gases believed to have existed in the atmosphere more than four billion years ago—the aforementioned methane, ammonia, and so on. Because lightning also is believed to have been frequent in such an atmosphere, and intense solar radiation also, considering that there was not yet an ozone layer, the scientists shot electricity through the water and gases. What they found was that, within a matter of hours, simple organic molecules like amino acids—the building blocks of proteins—were forming in the water.
One objection to this idea is that the same ionizing forces that were breaking up simple molecules to form more complex ones would also break up the complex ones long before they were complex enough to participate in the earliest life. One solution to that objection was that some of the complex molecules would sink far enough below the surface, possibly under mud, that they would not simply be broken up again by the next lightning strike.
Considering that this sort of organic molecule generation was occurring all over the planet for millions of years, it does not strike me as improbable that sooner or later, somewhere, some primitive proto-life would have occurred in a pond or lake or some such. The first proto-life may have been nothing more than a self-replicationg protein, a simple protein or group of proteins that would act as an enzyme or enzymes to produce more of the same. Cells would not have existed yet, though they also could have formed when lipid membranes evolved—and lipid membranes also can form spontaneously in a broth of organic molecules under favorable conditions. Anyway, it is plausible, at least, that the earliest life was a kind of anaerobic metabolizing broth in a pond or lake somewhere. Some scientists have hypothesized that the entire ocean at the time could have been a dilute solution of organic molecules, along with the minerals and everything else.
At any rate, the most primitive real life on earth at the present time, or rather in the eighties when I learned about it, is the Archaebacteria, which are anaerobic. Free oxygen is poisonous to them. So it does make sense, from an evolutionary point of view, that the most archaic life forms on the planet would be adapted to an atmosphere resembling the early earth. For more than two billion years the earth’s atmosphere contained almost no free oxygen, and the only life forms were anaerobic bacteria. When I was in school I was taught that it wasn’t until only a billion or so years ago that the atmosphere became aerobic, that is, it contained free oxygen rather than “reduced” carbon dioxide; though it seems that the date was pushed back after further discoveries were made, much like the date for the earliest humans and the earliest civilizations are pushed back when paleontologists and archeologists find something new. The earliest fossils known to science are around four billion years old, and they are exclusively of anaerobic bacteria.
Eventually though, maybe as far back as two billion years ago, there came to be enough oxygen in the atmosphere, probably due to photosynthetic bacteria, that aerobic metabolism could evolve and function. Aerobic metabolism is MUCH more energy-efficient than anaerobic: without oxygen one glucose molecule produces only two ATP molecules, the main ready energy source in a living cell; once aerobic metabolism came along, one glucose could generate thirty-two molecules of ATP, if I remember correctly. So once free oxygen is available to life, and aerobic respiration (with the TCA cycle taught in biochemistry classes for decades) has evolved, then cellular metabolism becomes 16 times more efficient. This is off to the races for organic life on this planet. It began an explosive next stage.
Shortly after there came to be free oxygen in the atmosphere (and this can be tested empirically through chemical analysis of the oldest rocks), nucleated cells—eukaryotes—evolved, or somehow came into existence. As I mentioned above, a cellular biologist who was also a creationist claimed that there is no way to explain how eukaryotes came to be, though there is plenty of evidence that nucleated cells with organelles like mitochondria and chloroplasts began as bacteria with parasitic or symbiotic bacteria living inside them. So a mitochondrion or chloroplast is a vestigial symbiotic bacterium—and to this day they have their own vestigial DNA. Apparently it became of great survival value to bacteria to have proto-mitochondria living inside them, as the mitochondria are the organelles that use (mostly toxic) oxygen to produce ATP at a very elevated rate over anaerobic respiration.
For some reasons unknown to me, nucleated cells and aerobic respiration facilitated the origin of multicellular life, dating back to perhaps one and a half billion years ago. It apparently took a few hundred million years for single-celled protozoa to evolve into multicellular life like primitive animals. An intermediate stage would be colonial protozoans, which still exist today; one of the best known is a pond alga called volvox. Even a sponge is a kind of protozoan colony, with no advanced organ systems: I have read that a live sponge can be pureed in a blender, and that afterwards the surviving cells with coalesce and re-form into a living sponge again.
The earliest animals resembled sponges, flatworms, sea anemones, and so forth, which evidently did not evolve into higher life forms than, say, jellyfish. But once nematode worms evolved a body plan that could evolve into more complex forms (with a mouth at one end and an anus at the other), a new stage of development became possible. Even so, for another billion years there was little if any life more complex than simple worms.
In our video the venerable ajahn mentioned the Cambrian explosion, which occurred around five hundred million years ago and lasted maybe twenty million or so. He claimed that Darwinian evolution through mutation and natural selection could hardly account for such an explosion in life—almost all of the phyla of animals alive today, including mollusks, annelids, arthropods, and chordates, arose during the Cambrian explosion. Although called an “explosion” it still took many millions of years. This “explosion” included many bizarre animal forms that appear to be “experimental” or prototypes for animal groups that just didn’t strike traction evolutionarily and became extinct. In fact most of the major groups that evolved or appeared during this relatively short period of time became extinct, and of course ALL of the species that are known to have appeared at that time no longer exist.
My explanation of the Cambrian explosion is that the necessary conditions for the development of complex animal life had suddenly become optimal, although aside from oxygen in the atmosphere and the relatively “recent” evolution or appearance of protozoans I am not sure what those conditions would be.
We are already eight-ninths of the way through the earth’s history with the Cambrian explosion. By the time the first dinosaurs appeared, around 250 million years ago, we are down to seventeen eighteenths, or almost 95% of the way through the history of earth. I may as well mention that the age of dinosaurs began with the Permian-Triassic extinction event, which caused most of the species on the earth, both in the oceans and on land, to become extinct. It is the greatest extinction event known to empirical science, at present. The prevailing theory is that volcanic activity in what is now Siberia caused a huge coal field to catch fire, resulting in massive amounts of CO2 being emitted into the atmosphere. This resulted in a greenhouse effect that warmed the oceans, and also CO2 being absorbed into the seas and becoming carbonic acid, which acidified the oceans and dissolved the calcium carbonate shells of a great many of the animals there. More species became extinct in the oceans than on land. This cataclysm that started the age of dinosaurs was a much bigger deal than the meteor impact that ended it.
I suppose I should mention another great evolutionary transition that occurred not too long after the Cambrian explosion, and that was the evolution of fishes into land-dwelling amphibians. It is counterintuitive, but nevertheless true, that lungs in a bony fish are an archaic trait. Bony fishes may have evolved in fresh water that could go stagnant and anoxic, and so they developed the ability to breathe air early on. The sturgeon is a good example of a primitive bony fish, a “living fossil,” that nevertheless has at least one lung (I was unable to find the exact number in a brief search on the Internet). The lungfish, closely related to the amphibians, is also a primitive bony fish, not an advanced one. The more “derived” groups of fish have a swim bladder, mainly a buoyancy organ, which is evolved from the more primitive fish lung. I may as well also point out that a tadpole, or larval amphibian, is essentially a fish.
Anyway, mammals evidently didn’t evolve until a little over a hundred million years ago, primates (descended from an arboreal shrew-like creature) maybe 85 million years ago, apes maybe twenty million years ago, and humans as recently as two million—although of course that depends on where one draws the line: if only Homo sapiens is considered to be truly human, as opposed to any member of the genus Homo, then humans have been around only a hundred thousand years or so, or .002% of the history of the earth.
As I say, the fossil record demonstrates very clearly that evolution has occurred on this planet; and only religious dogmatism or sheer bloody-mindedness can deny this. So the real debate is HOW evolution occurs. Personally, although organic evolution along Darwinian lines makes perfect sense from a materialist point of view, I am not a materialist, which causes some ambivalence. I suspect that something along the lines of Schopenhauer’s Will may still be a factor, possibly the main factor if physical matter cannot exist without a conscious mind perceiving it. But for whatever reasons, there is an elaborate fossil record going almost all the way back to the formation of earth which bears out the fact of organic evolution, or at the very least the change of life, usually from simple to complex, over the course of a very long time.
In conclusion, I would say that there have been some major revolutions in the history of life on earth. First, the beginning of life itself. Second, the beginning of aerobic respiration and the pretty much immediate development of nucleated eukaryotic cells. Then, shortly thereafter, the advent of multicellular life. I suppose the next major revolution may have been the arising of recursive self-consciousness and symbolic thought in human beings, though the evolution of the human brain (or whatever a non-materialist would say is the cause of our intelligence). And as I mentioned in the video, I suspect the next great revolution may be the transition from organic life to mechanical “life” and AI.
As I mentioned at the beginning of all this, this account of the history of life is based on standard scientific theory. It is just a theory; but then again, it is one with a huge wealth of evidence, unlike any competing theory that I know of. And to this day I suspect that intelligent non-Christians in the west who disbelieve in evolution do so out of a lack of understanding of it, or else it has simply become fashionable to doubt it. But the evidence is there, sufficient evidence anyhow to make some version of Darwinian evolution the default theory to be disproven. And just saying “Science can’t explain everything” is just not good enough.
|on the right is a fossil ammonite that I got for my birthday...|
|...they were the dominant oceanic life form before fishes arose|