3) The post-Cambrian period—the most recent half-billion years—is of course the most interesting because it’s the period in which our favorite species arose…along with, incidentally, all those other animals and plants whose descendants we’re familiar with today. To flesh out this chronology I will presently list and describe in brief summary the few major categories of life that evolution has brought into being during this most recent 500 million post-Cambrian years. More details can be found in biology textbooks, except those designed for religious schools that still teach children the Adam-from-mud theory.
Although this chapter’s brief march through these last half billion years of life evolution may zigzag a bit, it is intended to convey a sense of passing time, of logical sequence from long ago to now. A sampling approach is necessary because telling the entire story is quite impossible in the space available. One sometimes encounters an estimate that the millions of species alive on earth today represent only two percent of all the species that have ever lived on earth, the other ninety-eight percent having gone extinct. It’s probably true, though global overheating will soon prove that percentage too low.
In what follows, I hope to convey a tangible feel for the continuum of more primitive, less complex creatures slowly but ever evolving into less primitive, more complex creatures. And in all of it, with a few backward exceptions, the overall trend is always upward, to something higher—an upward “Urge” we evolving creatures all feel, toward which we are drawn, every one of us, human, skunk and bug. If this Urge were not self evident truth we would not bother to get out of bed or go to work or earn learned degrees. And then there’s the perfectly obvious: If earth had only one-celled creatures too dumb to do anything but breed for billions of years, and those creatures have now evolved into multi-organ complexes that can earn PhDs and speak in tongues, it can reasonably be said that this process has evolved upward, to something higher—never mind that materialistic scientists just hate to hear people state what is perfectly obvious to everyone but them.
Let us imagine (again) that the very first living thing, the first ever, started as some sort of molecule—a single, large inorganic (non-living) complex molecule consisting of numerous atoms coincidentally bound together by evolutionary chance. Further imagine that somehow—we don’t know how—this one particular molecule “became alive” by originating the ability to 1) survive and 2) reproduce itself. Let there be Life. These two acts define life—it can now survive and reproduce, things a non-living rock cannot do.
This molecule consisted of multiple atoms—perhaps up to a million. Self assembling itself by attracting and attaching additional atoms—as molecules do, rather like tinker toys—this first life had somehow became a “living” molecule that was different from all the other self-assembling but non-living molecules. Its main difference from all those others was that it “just had” an urge to 1) survive long enough to 2) reproduce, to make more of itself. Where did this urge come from? That first “alive” molecule felt this inner urge—the same urge we all still feel today, just about every day—that all the other complex molecules did not feel, could not feel, because they were not alive. Survive; reproduce. This is life’s mandate. By what means this small but important transition occurred we may never know, but of a certainty it in fact did occur because here we are.
To speculate, maybe that first molecule became alive because it passed some sort of complexity “threshold,” a tipping point between not-alive, then alive. Formation of celestial bodies is rather like that—things “tip”: galactic dust has to condense to a certain complex density before it can be called a galaxy; stars must graduate to a very major density before they get hot enough to ignite and become fusion furnaces.
It is reasonable to be speculating at the molecular level here. As noted above, we tend to regard viruses as more alive than not-alive, but this is a questionable assumption since they can’t survive on their own. A virus is nothing but a very complex molecule, so maybe the first life on earth was a proto-virus, a distant ancestor of these modern viruses which afflict so many people with a septic-tankful of viral diseases. Doctor, why are my sinuses stuffed up? You have a virus—it’s not biotic so an antibiotic won’t help, just act normal and your body will soon heal itself by throwing the invader out, probably.
This particular molecule was the first of its kind to be “alive.” Maybe that one first molecule was the only one, and we’re all descended from it. If so, “life” came into being only that one time, in that that first living thing. Or, an equally possible alternative, perhaps a dozen such molecules independently and redundantly did the same trick at a dozen times and places around the earth. Maybe a million—who knows?
Somewhere down the road these early-life molecules began attaching themselves in linear strings that would one day be called DNA, but we don’t know when or why this DNA-making process began. It had to be after that first molecule had reproduced itself into two more living molecules. Today, after some 3.5 billion years of life evolution, the human genome consists of over three billion DNA base pairs organized into 23 chromosomes, all neatly packed into the nucleus of every last human cell. Fantastic—what kind of imagination could dream up such incredible scope of evolutionary design? If you don’t believe in evolution because you know God made Adam from mud, or if you absolutely know there is no God and you reject all such ignorant superstition, the real facts about evolution of DNA suggest that God is a lot smarter than both of you.
After uncountable eons and generations, that alive molecule—the most primitive form of life—would eventually further evolve far beyond mere complex molecules, and become the larger, considerably more complex structures we call “cells.” Unlike energy as atoms joined into molecules, cells were much larger than any molecule, and distinguished by having an “outside” and an “inside” with all manner of complexity on the inside.
Thus evolution continued its endless work of changing everything. Then at some much later point in pre-Cambrian history, due solely to chance as always, these single cells “separated” themselves by evolving in two different directions that wound up as two broad new categories of living things: cells became either prokaryotic or eukaryotic.
Prokaryotes: unicellular simple life
The simpler (less complex) of the two are the prokaryotes, which include bacteria and “archaea.” The insignificant difference between bacteria and archaea is too obscure to hold most people’s attention (having to do with lipids and peptidoglycan, or some such). Prokaryotes’ little one-celled bodies do not possess a nucleus—their DNA just floats around in the fluid (cytoplasm) that is the main substance in their little one-celled bodies. Prokaryotes can—of their own free will—“decide” to wave their “flagella” (tiny hair-like appendages) in order to move toward food or away from danger. This ability to choose is pretty intelligent when you think about it—honoring, as it does, life’s mandate to survive long enough to reproduce. Nobody knows how they know which is food and which is danger, but they do—and this prettysmart “behavioral” ability to choose, to decide to take action to survive and reproduce, is a distinctive difference between life and non-life.
Stentor roeseli, a one-celled protozoan prokaryote common in groundwaters around the world today, spends its interesting life mostly attached to drifting algae and using its minute cilia to sweep food into the mouth end of its nutrition tube. A 2019 study at Stanford University found this simple prokaryote displaying a bag of tricks for avoiding danger (i.e., survive). Though lacking any central nervous system, it can decide which tricks to use, and it can change its mind. When experimenters released irritating chemicals upstream, S. roeseli tried to evade them with tactics that included first bending away, then reversing cilia action to spit foul water out of its mouth. When experimenters persisted, it further responded by contracting its body to spherical shape, and finally by releasing and floating away. Assaulted by a different irritant, pulsed plastic beads, it tried a variety of avoidance movements in varying sequences before detaching as a last resort. With survival behaviors clearly employing tactical choices, this primitive life form has—obviously—consciousness and intelligence. Maybe not as much as Donald Trump, but both evidences of life are there. We shall choose to return to this topic later.
Eukaryotes: multicellular simple life
I should mention right up front that if you are reading this page, you are a eukaryote. The eukaryotes are for the most part more complex than prokaryotes, in that their bodies consist of multiple cells (though the group does also include certain unicellular algae, fungi and “protozoa”—microscopic early animals such as amoebas and ciliates). Multicellular eukaryotes are distinguished by the organ-like interdependency among their multiple cells. At this point they are also called “organisms” because their parts depend on each other, and they must remain attached in order to stay alive. Since human beings arose on the eukaryotic line we have, for example, interdependent heart, lungs, liver and brain, any one of which without the others is useless for keeping you alive. Multi-cellularity apparently has evolved independently many times in the history of life.
Eukaryotes became increasingly complex with passing generations—the way starry space and most life forms do as they evolve—unlike the prokaryotes which have stayed simple, so far. Some simple prokaryotes happen to live in colonies, but not for mutual assistance—each unitary cell must conduct all its own life processes just to survive. In contrast, even the dumbest multi-celled eukaryotes, such as Congressmen, contain cells that depend on other cells to survive—a most basic principle of mutually interdependent life that eludes many Congressmen. And thus complex life emerged. Interestingly, most multicellular eukaryotes retain a prokaryote-like unicellular stage in their life cycle (not unlike the way all human fetuses develop and then un-develop a tail). In mammals the equivalent stage is the gametes or sex cells, known as sperm in men and ova or egg cells in women, which join during coital fertilization to form a new organism known as a baby.
Because eukaryotes can assemble/organize themselves into quite large groups, three types of them are candidates for the earliest slightly-more-complex life forms on earth. One is the fossilized microorganisms found in and around hot thermal vents on deep ocean floors. A second is stromatolites—usually found as hummocky mounds of column- or dome-shaped sedimentary rock that was deposited in successive layers by cyanobacteria, an early form of photosynthesizing life that lacked a DNA-packaged nucleus. The third is mineralized fossils of microorganisms found in the Pilbara craton (old, hard continental rock like we met in New Madrid) area of far western Australia.
All three types of eukaryotes are in the general range of 3.5 to 4.2 billion years old—the higher figure being not all that long after the hot young earth had finally formed its hardened crust. Pinning down exactly how many years ago they flourished is of course a risky business, indulged in by certain scientists who enjoy arguing (i.e., most of them). Of interest for our purposes, we see that these simple forms of multi-cellular life—all three on the verge of becoming more or much more complex—could do their evolving down in the deepest sea, up on the shallow sea edges, and way further up on the land. Pretty flexible. This broad distribution and obvious flexibility is probably important when considering the “explosive” rate at which ancient eukaryotes evolved into numerous more complex life forms during that brief 55-million-year Cambrian period.
Complexifying life’s mandate to reproduce: The invention of sex
Early single-celled living things met the reproductive commandment by simply cinching in at the middle until the middle separated and one cell became two cells, both fully able to do the same trick again when they somehow knew the time had come to do it. Nobody knows how they knew, but they did—this is called intelligence. To this very day many of our little unicellular relatives (we cannot yet call them brothers and sisters) use this simple way of reproducing, delighting student microscope users everywhere. They will never know the coital refinements enjoyed by their more complex eukaryotic cousins.
Among the latter, somehow, sex evolved—some say to increase the sum of happiness, but there’s no proof of that. Nobody really knows how sexual bi-genderism came about, but as you would suppose there are some learned theories. One is that two-gendered life at its most primitive originated when an organism with a damaged DNA section repaired itself by pushing its way through a similar organism’s outer membrane and, once inside, replicated the damaged section by copying from the similar organism’s undamaged DNA. No one can explain how this early life form thought up the idea of doing such a sophisticated thing, since we higher-intelligence hominins have ourselves learned how to copy DNA only in recent years—but it is a fair theory and is treated as respectable in some quarters. For that matter, we can’t explain how DNA replication is so extremely sophisticated in the first place. The whole business could just as credibly have emerged due to an evolve-survive-and-emerge urge built in by a Godly Algorithm, or something.
In any case, many living things, both plant and animal, thereafter “became” divided into hers and hims. All of them thereafter obeyed their compelling reproductive urge through the delightfully varied ways of scattering eggs and sperm (salmon), seeds on the wing (milkweed), or conjoining bodies in ecstatic sexual union. If you want your female holly tree to bear lovely little red berries, don’t forget to plant a boy holly nearby to please her.