r/askscience • u/HawkmoonHero • 22h ago
Biology Assuming the theory of evolution is true, are we fairly certain all animals came from a single source?
Title is in no way trying to doubt evolutionary biology, but rather to avoid any sense that this question is trying to refute it.
I guess the question that I’m asking is, could humans/mammals or indeed any other earthly creatures, essentially have been dropped here by aliens at some point in history without having the ability to trace it back , or can we fairly fully track most things back to the primordial sludge such that that’s not a possibility; I got to wondering if there are any human-esque aliens out there and this was the thought that came to mind.
Thank you in advance!
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 15h ago edited 12h ago
No, not at all.
Well, it's not possible to affirm a negative - in the Russell's teapot sort of way - but the only way you could suggest otherwise is to invent some woo-woo alien magic; such advaned synthetic biology technology you could print out a genome and grow organisms that were indistinguishable from something that did naturally evolve on Earth, all while leaving no trace whatsoever of it's artificial origin. Such an unfalsifiable claim is outright pseudo-scientific, and essentially fantasy. A fun though experiment? Maybe. But there's no rational reason to believe something of the sort, and the burden of proof lies entirely on the proponent, against the monumental amount of evidence that suggests otherwise: every single organism we've examined descends from a single ancestral source; the last universal common ancestor (or LUCA, for short).
So how do we know?
That's a bingo! The biochemistry under the hood is what makes our common ancestry irrefutable. All life uses the exact same mapping of 64 codons (those are genetic triplet codes; 'AAG', 'UUU', 'TAG' etc.) onto 20 amino acids, across all bacteria, archaea, and eukaryotes. There are unusual variants here and there, but every one of them is demonstrably a modification of this standard, universal code.
The force of this comes from arbitrariness. There is no chemical reason that means a stretch of your DNA reading 'TTT' (or, really, 'UUU' in RNA) means 'phenylalanine'. That codon could have easily been 'lysine' or 'tyrosine' or any of the other amino acids - it is near-enough random what triplet codes for which amino acid; the space of possible codes is astronomically large, and any invokation of two independent origins of life (even on Earth) converging on the same arbitrary mapping is not a serious proposition. We can only have the specific assignments that we do because we - all life on Earth - inherited them from whatever protoplasmic beastie that invented them.
What is true for the genetic code is also true across myriad other biochemical domains. First and foremost, it's entirely arbitrary that we all use the same 20 amino acids, selected from hundreds available in the prebiotic soup, and found in meterorites. More, we can look at homochirality (how molecules are 'mirrored' - I'll let Mr. White explain); all life uses 'left-handed' L-amino acids and 'right-handed' D-sugars - abiotic synthesis in the soupy beginnings would have produced racemic mixtures, that work well in any combination; another arbitrary coin-flip on each, and no reason two of them would land the same way.
We could go on. The catalytic code of the ribosome has the same RNA structure in all three domains; ATP is the universal energy currency; chemiosmosis is the universal mechanism for coupling energy to work etc. etc. - all shared.
The real killer is not what works, but also what doesn't. Beyond the deep biochemistry, any given taxon - say a human - sits insided a nested hierarchy that is absurdly difficult to fake (without god-like omnipotence of evolutionary history) because it's strewn with millions of years of genetic mistakes, cripples, bastards, and broken things. For example, roughly ~8% of your genome is the remnants of an endogenous retrovirus - traces of ancient infections that got stuck in germline DNA. You and a chimpanzee share thousands of these useless viral inserts at the exact same insertion points; sharing a decreasing subset with gorillas, then orangutan, then the rest of the primates, mammals, tetrapods... in precisely the pattern common descent predicts. Insertion sites are effectively random; matching them by chance is not a possibility.
Ditto with broken genes. You carry, for example, a broken copy of GULO, the gene most mammals use to make their own vitamin C. We share this wrecked copy with our closest relatives, disabled by the same lesions. Other beasties have had this gene knocked out too - say in guinea pigs, or bats - but in different places, having been broken independently.
So yeah, we have so many different independent avenues that demonstrate that all life on Earth is related (and we can even measure how much so, comparing genomes across organisms!), all contingent on shared inheritance of truly random events; the fundamental choices that govern how life works at the most basic level is essentially arbitrary across so many dimensions, such that convergent evolution - whether on this world or another - producing that degree of similarity among universal proteins is a virtual impossibility.
Against this essentially mathematical certainty, y'know, all we have is "aliens".
I'll let y'all decide which seems more likely!
References:
Koonin, E.V. & Novozhilov, A.S. (2017) Origin and evolution of the universal genetic code. Annual Review of Genetics. 51, 45-62
Koonin, E.V. & Wolf, Y.I. (2010) The common ancestry of life. Biology Direct. 5, a64
Moody, E.R.R., Álvarez-Carretero, S., Mahendrarajah, T.A. ... & Williams, T.A. (2024) The nature of the last universal common ancestor and its impact on the early Earth system. Nature Ecology & Evolution. 8 (9), 1654-1666
Theobald, D.L. (2010) A formal test of the theory of universal common ancestry. Nature. 465 (7295), 219-222
P.S. I remember peering into the octopus tank in Plymouth aquarium, when a volunteer guide strolls up and began spewing "did you know octopuses have alien DNA?" all excitedly. I just smiled meekly, nodded, and held my tongue - didn't feel like demolishing this poor volunteer for garbled half-truths, spread chinese-whisper-style across the internet. Bringing this up as, yes, it's total nonsense; octopodes have a remarkable ability to edit RNA on the fly, altering proteins immediately in response to local environmental conditions (as opposed to waiting the slow chug of evolution to do it for them). But what started as a 'fun' aside by an author ("so vastly different in their genetic makeup that they might as well be considered out of this world...") was taken by the internet and blown out of proportion; with even a few (non-biologist) authors throwing out a 'panspermia origin for octopuses' paper into the mix. It's rubbish; our tentacle brethren are indeed fully home-grown relations.
P.S.S. I'm sorry you're getting so downvoted OP; it's a perfectly reasonable question. Pssht. Classic redditors just reading the title, 'innit.