The timing of the delivery is what's important here. These building blocks, organic matter, and water would have been depleted in the proto-Earth due to Solar irradiation. There needs to be some mechanism that delivers these ingredients from the outer Solar System. Bombardment by smaller rocks makes the most sense, and was likely triggered by the migration of Giant Planets, leading to a period of heavy bombardment (on a bare Earth -- no oceans, no volcanoes).. https://en.wikipedia.org/wiki/Nice_model

I guess it depends on how you define life, and whether we'd even recognize it when we see it, assuming we're looking in the right places.

I'd also imagine that any type of chemistry that harvests energy from the environment is liable to find itself as a food source at the bottom of the food chain now that earth is teeming with life.

I think that self-replication, and ability to harvest chemicals and energy from the environment to make more of what you're built of, is the point of complexification of chemistry that is best considered as the most primitive form of life. From there you can go on to things that are capable of encoding structure and more complex chemical factories.

I suppose one signature of these earliest type of "emergent life" chemistries would be localized concentrations of things like these nucleobases that we know are the building blocks of life as we know it, but there may be other types of self-replicating chemistries that emerge too, that don't lead anywhere.

I agree. If we knew the mechanism for how life started we'd probably be doing the experiments to prove it. There are theories and experiments that suggest some life-like processes can happen with inorganic compounds, but they require a lot of squinting and a bit of imagination to connect with our own origins. And there's a big difference between experiment and nature. On the one hand, we have people trying to make it happen, while on the other hand, it apparently already happened once, without anyone even needing to be around.

> Wouldn't a couple asteroids be the literal drop in the ocean in comparison?

Actually most water on earth probably came from asteroids, so they are the entire ocean! They would also have brought a lot of frozen methane and ammonia, so most of the chemicals necessary for terrestial life.

When the solar system was forming, the protoplanetary ring of cosmic dust would have consisted of heavy elements (some essential for life, such as phosphorus) closer to the sun and frozen lighter elements further away. The heavy elements would have combined into the early rocky earth, and as the other planets formed and orbits stabilized the icy asteroids from further out would have been flung around and impacted the planets.

The major flaw in Panspermia is that it all had to start somewhere without Panspermia. If it did that there, why not here?

My layman guess would be that shortly after formation Earth was just a ball of lava that destroyed every organic component so when the surface solidified it was sterile.

https://en.wikipedia.org/wiki/Late_Heavy_Bombardment - possibly?

Comets is where many astronomers have long thought the ocean came from. Comets are literal drops in our ocean. LOTS of comets. The atmosphere and the Earth at large would have been very different, and being bombarded by many giant space snowballs (along with asteroids) would have contributed materials. The missing part is, um, missing. We still do not know. However, these samples contained building blocks, not actual self-replicating RNA. That might seem like nothing, but before this discovery, we thought they only contained one ingredient.

Well the competition might be too fierce for any new life to develop

Why are you assuming couple of asteroids? Life first appeared 3.5 billion years ago. The frequency of an asteroid impact on Earth is ~500,000 years.

> I'm no expert but this sounds strange.

A cynic might suggest the theory might exist because nobody could figure out how life got started on its own on earth.

The thing is I've never found the asteroid theory particularly satisfying either because it simply inserts another abstraction layer, explaining the problem away rather than explaining it.

That's not to say it's wrong but, in its current incarnation, it's just a bit meh.

I suppose perhaps that's part and parcel of it being a very hard problem to solve.

This theory is called panspermia [1] and it has several alternatives. One of the most extreme is that in the very early Universe, these building blocks could spread easily because the ambient temperature of the Universe was significantly higher than it is now. This isn't the most popular version.

The most popular is that asteroids and other interstellar bodies spread the building blocks, be it anywhere from amino acids to more complex building blocks. As evidence of this, there are hundreds of surviving asteroids on Earth that have been positively identified as having coming from Mars, which is pretty crazy because that basically takes a violent impact throwing debris into space and it making it to us many times over.

Part of the evidence for all this is how soon after the Earth formed that life appeared. We have positive evidence that this only took a few hundred million years. That's kinda crazy if you think about it. Also consider that the oceans likely came after the EArth formed.

Our galaxy is over 10 billion years old. The Sun is less than 5 billion years old. So that's 5+ billion years for stars and Solar Systems to form, evolve and die before the first fusion reaction in the Sun. Some of this needed to happen just to form heavy elements that are relatively abundant. Even that's kind of crazy. Heavy elements like lead, uranium and gold take relatively rare and violent events to eject material into space and make it to us. So what else made it to us?

[1]: https://en.wikipedia.org/wiki/Panspermia

> One longstanding theory is that life first began on Earth when asteroids carrying fundamental elements crashed into our planet long ago.

That theory is bullocks. When an asteroid enters the atmosphere and crashes as high speed on the surface, you get a huge amount of energy that creates an explosion and a destruction of most complex chemical material in the process. It's no mistake that these kind of impacts are counted in the same range as multiple atomic or hydrogen bombs.

There are 2 distinct kinds of claims made about the role of meteorites fallen on Earth whose origin is in such bodies like the Ryugu asteroid.

One claim, which is likely to be true, is that in the beginning the Earth had a lower content of volatile elements, e.g. hydrogen, nitrogen, carbon, oxygen and sulfur, than today. The reason is that Earth has condensed at a high temperature, being close to the Sun, and those elements would not have condensed.

Later, the Earth has been bombarded by a great number of asteroids formed far from the Sun, which were much richer in H, C, N, O and S, and this bombardment has provided a major part of the chemical elements required for water and for organic substances.

A second, different claim, which is almost certainly false, is that this bombardment of the Earth has provided not only the raw chemical elements, but also pre-synthesized organic substances, like amino-acids and nucleobases, which have taken part directly in the origin of life.

This second claim does not make sense. The meteorites rich in water and organic substances are extremely easily vaporized during atmospheric entry or during the impact with the surface and their content of organic substances would decompose.

Even if we suppose that some falling bodies were so big that parts of them survived until the surface, any organic substances thus brought on Earth could not help in any way the appearance of life.

Any form of life would need a continuous supply of such substances, otherwise immediately after consuming the few molecules adjacent to it the life form would die without descendants.

Life can appear only in a place where there is a continuous supply of energy and it can use only chemical substances that are continuously synthesized in abiotic conditions. It cannot appear based on sporadic events, like the fall of a meteorite, which would also destroy anything at its place of impact.

Such places where energy is available continuously and there are also the substances from which complex organic substances can be synthesized through catalysis by various minerals, mostly metallic sulfides, exist both on Earth and in other places in the Solar System. These are the places where either volcanic gases are released or similar gases are produced by the reaction of water with volcanic rocks, in hydrothermal vents. As far as we know, those are the places where life must have appeared, because all the necessary ingredients exist. The only mysterious part is how it has happened that a correct combination of the mineral catalysts required to synthesize all the needed organic molecules happened to be located in close proximity and in the right sequence.

Today, even if such places still exist on Earth, life could not appear again. First, the oxygen from air would destroy any substances thus formed, and even where oxygen is missing the ubiquitous bacteria would consume any organic substances that could form abiotically, preventing their accumulation and the formation of any kind of structure from them.

Admittedly, am layman, have only heard numerous sciencey folks talk about it, but we've found all these basic components in space already, naturally occurring, and while we've never to my knowledge recreated actual, genuine abiogenesis, we have observed every process required for abiogenesis to be a reasonable explanation for the origin of life.

As to your question on we should see the formation of new life everywhere, well, if we looked hard enough we might? The answer is competitive exclusion. Abiogenesis would've occurred on a remarkably clean earth: any life now emerging from the proverbial space dust is both almost certainly not preconfigured for this biosphere, and is instantly drowning in competing microorganisms that are. Anything that does form is likely quickly killed either by natural forces or competing organisms. Meanwhile, our life goes everywhere: We've found living bacteria on the outside of the ISS!

Yep, you are 100% correct. In fact, it is much more likely they were originating on Earth itself than a random hobo asteroid.

> The missing part is how do they form self-replicating mechanisms capable of evolution.

Well, there are some missing parts, yes, but RNA can self-replicate already; at the least some RNA can. Ribosomes also contain RNA so its is a ribozyme.

The missing part has been conducted in other experiments. I don’t have time to give you some papers, but nucleic acids can self assemble into long chains under the right condutions. No polymerase enzyme is needed.