Even though we see a vast array of different forms of life, the basic DNA building blocks are the same in all life. Many different kinds of life share the same genes, like plants and animals. We know the variety happens through evolution when species diverge. But why didn’t this kind of variety happen to create different basis for life in the first place? The early Earth was covered in water and organic compounds. Why didn’t primordial ooze in different ponds develop totally different kinds of life with completely different genes or even non-gene based life? Even if the base A, C, G, and T bases in DNA are true base components of life, why do the same arrangements of them show up in genes that are the same in all different kinds of life? The consistency in DNA makes it seem like life originated from a single location.
How life originated is still, and may always be, a mystery.
One possible answer to your question is that lots of different types of life did emerge in the early Earth, and DNA-based life outcompeted all the others.
The beginning of life was an extremely rare occurrence. It didn’t necessarily develop in various ponds over the face of the planet. All it needed to do was develop once, in one location. If it did, in fact, develop more than once in more than once place, the DNA-based organisms won out.
It’s not just that all life uses DNA with bases A, C, G, and T. Maybe there are other possible genetic molecules, and maybe there aren’t, but in any event, we don’t have any way of counting how many there could be. But all life that we know of even uses the same encoding of those bases into amino acids. And we can count how many possible encodings there are, and that’s a ludicrously high number, so yes, it’s as near to certain as it’s possible to be that all life that we know of is descended from a common ancestor.
But as zimaane says, it’s quite plausible that life originated multiple times, but each would have regarded all of the others as a good source of raw materials. And whichever one happened to be first would probably have a big advantage in the competition, since however long the gap was between the first life and the second, the first life would have spent all that time evolving to be more capable.
Or, of course, maybe that jump from almost-alive to completely-alive is a big enough one that it did happen only once. We don’t know.
It might have. You have to remember that life on the Earth was probably repeatedly wiped out. You have the mars sized planet that hit the Earth and created the moon. There were multiple heavy bombardment periods, including the late one that might have been due to Mars getting hit by something the size of our Moon and nearly destroying the planet.
Our line of life was basically the one that didn’t get completely wiped out and also managed to spread over the planets surface. Even then, you had a variant poisoning the atmosphere with oxygen and wiping most of the earlier forms out (any that couldn’t adapt). Then you had mass extinctions that didn’t, quite, kill all life.
With all of that, you could have plenty of variants that simply didn’t make it and there is just no evidence for them now, after a billion or more years.
Life moves a lot faster than random chemical reactions.
Let’s say, for example, that random chemical reactions will produce a life form, on average, once every hundred thousand years. And let’s say that life forms can reproduce and fill up all of the available niches in an environment in ten thousand years. These numbers say that the first random life form will be the only one generated by random chemical reactions because that first one will take over completely before a second one appears.
If I may introduce a tangent:
Among geneticists, biologists, etc. … is there a consensus of opinion about whether or not other genetic molecules exist in the universe? That is: is life based on different “chemical” (?) bases altogether considered possible?
Tangent 2:
Let’s say we discover organisms on an exoplanet way far in the future. Algae-like, insect-like, tree-like, worm-like, higher forms, whatever. Upon testing, we learn that those organisms also are based on DNA with the same four bases as Earth life. Would that be a “just as we suspected” moment for Earth scientists from a 2019, or would that be considered a most unexpected “holy crap!” finding?
EDIT: I meant to stipulate that these questions are about carbon-based life, not necessarily about theoretical silicon-based life, or life based on a non-water solvent, or anything like that.
There’s also the theory I’ve seen posited that life is actually a fairly frequent occurrence but at this point any that comes into being is quickly destroyed by established forms.
But why wouldn’t we see each form of life finding it’s own niche just like we see with existing organisms? It’s not like there’s just one organism on Earth. From the smallest to the largest scale, there is a huge diversity of organisms even though they may be in direct competition with each other. If parallel life developed, it seems like it should be possible for it to survive in the same way that two different organisms based on AGCT DNA can survive.
For life nowadays, challenges typically consist of things like “existing despite the presence of a predator that wants to eat me”, or “existing despite a shortage of food”, or the like. But for the very earliest life, the big challenge was just existing, period. Take some of that first life, and put it in a Petri dish full of whatever chemicals it likes best as food, in a climate-controlled environment, and so on, and it’ll still struggle. And now along comes something that’s had a hundred thousand years of experience at that whole “being alive” thing.
The pond you mention is called the Pacific and the soup where life began is the surroundings of the black smokers or deep sea vents, some 2500 to 3000 meters below the surface.
Because any new life would be made out of tasty organic molecules that existing organisms would love to eat. And it wouldn’t have had billions of years to figure out ways to avoid being eaten like a protective coat or toxic chemicals. It would be like a newborn baby in a jungle full of hungry predators.
Consider this. The definition of “life” varies, but one trait that definitely IS a defining feature of living things is the ability to reproduce. Once you have a life form that can reproduce, is it any surprise that it would outcompete random chains of organic compounds that could, by chance, align in such a way as to allow reproduction?
If a bacterium stumbled across a collection of protein chains and organic compounds that happened to collect in a secluded hole in the side of a geothermal vent, the bacterium would proceed to consume the delicious organic materials and use them to produce a second bacteria. That will happen much, much sooner than said clump of organic compounds could develop the ability to reproduce.
Also, it is entirely possible that this DID occur at some point in history. There are a few competing theories for the origin of viruses, and at least one theory states that they have a common ancestor with us cell-based life, but that while bacteria and multicellular organisms became more complex, viruses instead became simpler.
But alternate theories include viruses forming from nonliving organics after cells already existed – a second abiogenesis that created parasites that prey on cells. Some theories even state that not only do viruses have an independent origin from celled life, but some viruses may not even share a common ancestor with each other, forming independently.
Now, if we found evidence that life elsewhere in the universe shares our DNA structure, that would likely point to panspermia as the origin of life. IE, either primitive life or its preassembled building blocks came to Earth from elsewhere. We do know that complex organic compounds exist on their own in space, including in the proto planetary disks that collapse into planets. It is definitely possible that this is the source of life, or at least of its building blocks.
In synthetic biology, it is possible to mess with unnatural base pairs and/or the genetic code and genome, so it is certainly considered possible.
But anyway, the way genealogy works, one can expect there to be a most recent common ancestor for all life on Earth, and estimate when this individual may have lived- billions of years ago (Wikipedia cites 3.5-3.8).
As for the question of present-day formation of new life from non-living chemicals (abiogenesis), in addition to the fact that Earth is now saturated with hungry living things (all honed by billions of years of natural selection into highly efficient survival machines), there’s also the fact that Earth’s atmosphere is (in large part thanks to life) drastically different than it was 3 or 4.whatever billion years ago when abiogenesis presumably first occurred on this planet. It may well be that current conditions (such as large amounts of highly reactive and corrosive molecular oxygen all over the place) are not really compatible with the spontaneous development of proto-living things from simple chemical reactions.
The truth is, we know very little about abiogenesis, so it’s hard to say with total certainty why it isn’t happening now, or even whether it’s happening now (with the resulting baby biospheres presumably being quickly eaten or otherwise dying off); we don’t really know for sure precisely what abiogenesis would even look like if we saw it.
Life on Earth is a *lot *older than the Pacific Ocean.
This is a good overview. I’ll amplify:
Stephen Jay Gould argued for contingency as an important driver of evolution. Basically, luck or chance often kills off a genetic line, allowing a different genetic line to expand its niche, but it might never have happened without the accident. A thought experiment: how successful would very early hominids have been in a world still populated by Tyrannosaurus Rex and Utahraptor? But those guys were hit by a rock, and our ancestors caught a break.
Here’s a little .pdf summarizing Gould’s thinking:
https://www.cell.com/current-biology/pdf/S0960-9822(06)02143-9.pdf [WARNING - .pdf]
Are there any thoughts about whether there was a first DNA-0 which reproduced, or whether there was some place which had the conditions necessary to pump out copies of the first DNA over and over from organic material? It would be like the difference between putting a single bacteria in a petri dish where it reproduces versus having environmental conditions just right to produce a certain crystal (like salt crystals).
DNA was certainly not the first molecule able to reproduce itself. It was probably preceded by RNA, and RNA by even simpler molecules. The triggering factor most likely was a molecule that found a way to reliably reproduce itself. The initial conditions may have produced a number of copies of molecules that could reproduce themselves when conditions were right, but the one that could do so reliably would win the race.
This article suggests that more than one million different nucleic acid-like molecules could possibly store genetic information. It’s possible that DNA/RNA have some advantages that might make them the most suitable ones to store a genetic code, but others are certainly possible. And the article doesn’t address other possible systems that are not nucleic-acid type.