A few comments on various things people have mentioned:
The age of the Earth is estimated at 4.6 billion years. Geochronologists recently identified the oldest known piece of continental crust – a small patch of northwestern Canada dated to 4.055 billion years. So within the first 500 million years or so of the Earth’s existence, differentiation of molten rock into a crust, mantle and core was well underway (although continental crust continued to accumulate in volume for possibly another 1.5 billion years). Early large-scale bombardment of the Earth by dust and debris, including the Mars-sized impactor that is thought to be responsible for the Moon’s existence, would probably have resulted in the repeated destruction of early atmospheres formed through volcanic outgassing of the Earth’s interior. That bombardment is believed to have died down by the time the Earth was 400 million years old or so.
IIRC, the oldest possible fossil microbes are about 3.8 billion years old; some geochemical evidence suggests biological activity as early as 3.9 billion years. The early appearance of single-celled life forms poses a huge problem in understanding the subsequent evolution of life. Why would organic molecules be able to make the leap to single-celled organisms, complete with cell walls and genetic material (RNA or DNA), in maybe 300 million years or less, and then make no clear progress towards multicellular life forms for another 2.5 billion years or more? (The oldest known multicellular algae are just 1.4 billion years old. The oldest fossils of multicellular animals are about 600 million years old; molecular systematics studies suggest that they might have emerged as early as 1.2 billion years ago.)
The apparent slow tempo of microbial evolution through much of Earth history has provided some grist for the panspermia mill. If it’s so difficult for life to evolve beyond what we would consider a “primitive” stage, the reasoning goes, then surely life must have begun elsewhere and been brought here. The presence of abundant organic molecules in space offers support for the hypothesis, in that we know the materials necessary for Earth-like life are at least available (as several people have pointed out). Most of the mechanisms I‘ve heard discussed for the transport of life to this world don’t involve meteorite ejecta (sedimentary rocks, etc.) from other worlds arriving here, but rather comets. Comets offer some advantages in that the appropriate biochemicals and/or life forms could have been protected from the rigors of space by ice; cometary ice is already thought to have been an important contributor of water to early Earth oceans. If life required stable conditions to form, then it’s unlikely that any body belonging to the inner solar systems (i.e., the planets, their moons, and the asteroids) were the source, because their early history would have been as equally violent as the Earth’s.
The $64,000 question that Scylla raises is: just how hard is it for life to form from organic molecules? At the moment, no one really has a clue. Mixing a bunch of organic chemicals together in a “soup” doesn’t seem to encourage any sort of biological activity. Mineral substrates the replication of proteins or primitive RNA molecules is an attractive idea, but no lab experiments have yet succeeded in coaxing organic chemicals into life-like activity. Perhaps there is some other mechanism that no one’s thought of yet. The microbiologists will be scurrying around in their labs for a while yet, I suspect.
For the moment, let’s assume that we’re all dunces, and it’s actually easy to start life up from a bunch of chemicals once you know how. Then there’s no problem with Earth life beginning on Earth, but how do you do it under the aforementioned hostile conditions? One good possibility is in the dark, at the ocean bottom, where hydrothermal vents spewing nutrients could have been oases for chemotrophs. The water overhead would have provided a buffer against violent impacts early on, as well as facilitating chemical reactions to spur development and geographic distribution of organisms.
OK, you’re saying, that doesn’t explain why there are organisms with adaptations for conditions that surely don’t exist on the Earth now, but are more characteristic of space. One possible answer is that the traits we now see originally evolved for a different function, and accidentally provided some additional benefit that was perpetuated. For example, there is a hypothesis that biomineralization (the formation of shells) originated as a way to protect organisms from spontaneous encrusting of their bodies in an ocean that was supersaturated with respect to calcium carbonate. The fact that shells subsequently became useful for protecting organisms from predation was an added bonus that enhanced the survivability of the organisms that possessed it; so the ability to make shells was retained even after ocean chemistry changed and became undersaturated w/r/t calcium carbonate. Specialized traits in micro-organisms may not be any different; for example, Deinococcus radiodurans is resistant to radiation because its DNA contains a remarkable number of redundant sequences, giving the critter the ability to repair ANY damage to its DNA (caused by radiation or anything else).
Intensely salty, radioactive, dry, hot, cold, pressurized, and mineral-laden environments do exist in abundance on this world. Microbiologists are just beginning to appreciate the diversity of environments that Earth life is capable of surviving. The fact that organisms can survive in these environments is not sufficient evidence that they arose off this world. As I pointed out in my earlier post, it will be devilishly difficult to demonstrate where the mother-of-all micro-organisms arose. (A logistical problem also presents itself: how to analyze and compare the DNA of the millions of species of single-celled organisms to narrow the search down to the “most primitive” ones genetically. Also, what do we do with viruses?)
A few additional comments:
If life originated on this world in the first 500 million years of its existence, during the early bombardment period, DNA may not be the carrier of genetic material in life that subsequently developed off-world. DNA, and the process by which it is replicated, are complex biochemically and would have been a relatively “late” development. If proteins or RNA molecules were thrown from Earth and survived elsewhere, their subsequent evolutionary paths could have been quite different.
These were likely terrestrial contaminants. Components for space vehicles and satellites are constructed and assembled in clean rooms, but I don’t think the manufacturing sites filter for microbes.
See my comment above on adaptations. Early oceans may have been clement, relatively speaking, but they would still have been extreme environments compared to the world as a whole today. There is no actual evidence for micro-organisms surviving 230 million years in space; if this is Zubrin’s quote, I think he misspoke. It may be possible, but it has not been demonstrated (and would be very difficult to do, in any event). Alternatively, perhaps he was thinking of the oldest fossili
