Author's Note: Circumstances are conspiring to keep me from finishing this essay. Honestly, the past month, I don't know what the fuck I've gotten done. In any case, clearly I am not going to complete this essay the way I desire, and so, I'm just gonna wing it even more than usual, and let the pieces fall off where they may....
^^^Which set of molecules appears to be running the joint? The RNA based ribosome or the DNA strand?
This particular essay is nothing but rank speculation. Still, the origins of Earth and Life are lacking in a lot of data, subject to a lot of conjecture, so I don't feel too terrible generating what may be well just-so stories.
We deep sea denizens, living at the bottom of an ocean of air, rarely worry about radioactivity. The universe is just a hissing, spitting cat of radioactivity, and, like any playful kitten, loves to kill things just for the fuck of it.
Cute playful kitty has all sorts of different claws to accomplish this: EM radiation from UV on up, into the gamma region, but not beyond due to matter/energy interchange; relatively "harmless" alpha and beta decay from unstable radionuclides, and, of course, cosmic rays - nearly as fast as speed of light particles ranging from protons up to bare naked uranium nuclei (and maybe even, given relativistic time dilation, the occasional ununseptium nucleus). All of these types are usually brought under the rubric of ionizing radiation. Out in space, beyond low Earth orbit, this ionizing radiation sets a limit of perhaps a three months or so of unshielded human exposure before incapacitation or death.
We are sheltered from most space radiation, and a good thing too, but still almost every element beyond hydrogen can be rendered unstable with a good enough whack from a stray neutron, and so we inescapably exist in a ionizing radiation environment. But I'm getting ahead of myself...
Let's start at the beginning: 4.537 billion years ago, a proto-Earth formed out of a rapidly accreting disk of dust and gas, orbiting a proto-Sun. About 150 million years later, that proto-Earth, sometimes dubbed Thera, got rear-ended by a Mars-sized object named Theia. It wasn't a big collision, not a t-bone accident, but it was enough to create the Moon. Point being, given we don't have many rocks from the origin of the Earth, Moon rocks - and meteorites - tells us all we know.
(It should be noted that, when you look at depictions of the formation of the solar system, they give the wrong impression, the same way that those bathtub-drain renderings of wormholes and black holes, or those whole-lotta-empty-space depictions of neurons and synapses in the brain give the wrong impression. Wormholes and black holes do not look like water going down a bathtub drain, and neurons are packed cheek-to-jowl in a very crowded brain. And so it was with our solar system at the beginning, context is left out, namely the nebula which held the stellar nursery of hundreds or thousands of stars of which our Sun was but one).
Again, point being the composition of that cold molecular cloud, at the turbulent compressive front of a spiral arm, determines the type and amount of radionuclides we find on early Earth, and thus, Earth today.
And we don't know much about it. We used to think that a supernova must have gone off nearby to collapse the cloud destined to become the solar system. Now, we think not. (Although, to explain the ratio of aluminum 26 to magnesium 26, perhaps a supernova went off within our stellar nursery some half a billion years before the solar system formed).
We thought that we had identified the cluster of stars we came from. We were wrong. The nearby nebula that astronomers considered a candidate would have had to have flung the sun out with such force that all the planets would have been stripped away.
Lately, there is news that perhaps we have found a sibling star from that distant stellar nursery. If so, then the origin point of our cluster of stars could be anywhere, maybe even on the other side of the galaxy. Maybe someday we can create a "galactic genome" map of all stellar origins using elemental and orbital dynamics data, in which case we might find the origin point of the sun's birth, but in the meantime, we have only conjecture.
Regardless, here's what we can suppose about the conditions for life on the early Earth with respect to radiation. So long as the Sun was contained in the stellar cluster, the solar system would have been bombarded with intense ultraviolet radiation from massive hot blue stars. Earth, in turn, received copious amounts of UV from the young Sun as well. Couple this with a massive amount of unstable actinides (heavy elements in the uranium on up range - including naturally occurring plutonium) and you have an extremely hazardous and unlikely environment in which Life formed and thrived. There was, in fact, so much goddamned radioactivity that it's a wonder anything managed to hold together.
But here is something interesting, and not entirely coincidental about damage from ionizing radiation, Life uses the same repair mechanisms to cope with it as it does for oxygenation. Oxygen getting in where it isn't supposed to be create free radicals and superoxides, both types of compounds extremely obnoxious and unsuitable for cellular metabolism. Oxygen is almost as bad as it's next-door neighbors chlorine and fluorine. When cosmic rays, or suitably high energy EM radiation, tears into molecules and cellular structure like microscopic 50 caliber machine guns, it creates conditions similar to oxygen poisoning.
Coincidence? Maybe, but consider that most radionuclides do not form water soluble compounds in a reducing atmosphere as the early Earth supposedly had. But oxygenated water and air, even only locally, sets up ideal conditions for fission reactions to naturally occur.
Oxygen also produces ozone, which cuts back on deadly UV radiation. So, we have competing threats to life, nuclear radiation, and UV radiation. Life came up with a strategy for the latter in the form of melanin, an ancient and honorable molecule. So, what about the strategy for ionizing radiation?
It is interesting to note that some 3.87 billions years ago, DNA became the standard format for information storage. It is hard not to imagine RNA coopting and regulating proteins in the environment before then, but DNA has a fidelity of information transmission that RNA envies. Oh, true, information replication via RNa is quite robust, but the signal to noise ratio is much lower than for the more fragile DNA format.
So, what gives? What made DNA the preferred medium of transmission? My guess is, all the little cellular machineries needed a lot less physical error checking algorithms for DNA than RNA, and that extra metabolic energies could be devoted to other things, and so organisms using DNA got a bump up in the ratings. That's one part of guess...
But! What else happened around 3.87 billion years ago, class? Could it be the Late Heavy Bombardment? Why, yes. Yes it was. But the heavy bombardment may not have been nearly as heavy as planetary scars suggest. And further, the proposed mechanism - the scattering of Kuiper belt objects and a shift of Neptune's orbit further out due to resonance between Jupiter and Saturn - could have been a symptom of something else going on.
My wild speculation is the bombardment, and the migration of planets in our solar system, was the result of the baby Sun finally leaving its birth nebula, perhaps assisted by the passing of a nearby star or stars. Further, those stars that surrounded the early solar system were the older big blue stars that formed first within the nebula. The intense UV radiation flux from these stars is finally gone, and DNA, which readily absorbs UVB radiation, gets a break from that. It still has to deal with ionizing radiation, and the mechanisms for that must wait until sufficient oxygen accumulates for oxygenation coping strategies to develop. Unless... the oxygenation coping strategies are based upon some older metabolic tricks from the Hadean or early Archean lifestyle, in which case we are back to life thriving under very intense radiation conditions.
Well, there you go, not quite the essay I wanted, but it will have to do.
(Oh, the white people are vampires thing from the Part 1 essay? That was a sloppy inference based upon the fact that animals with less pigmentation seem to thrive better in radioactively contaminated environments.)