Yes, there was quite a fashion for checking the code of different organisms back in the 70’s. Thousands of organisms were checked. Of course none of that info seems to be available online these days, but I did find a couple nice charts of the code variants that have been found in nuclei and mitochondria.
As you can see, the variation is minor, one or a few codons not coding for the amino acids they usually do. There are no cases where the codon->amino acid table is effectively scrambled, or contains codes for nonstandard (ie not of the usual 20) amino acids. You’d expect such things if the code had arisen from nonbiological matter more than once.
No, we haven’t turned over enough rocks yet to be certain that all current earth life is the product of one biogenic event, but we have turned over enough to be darned surprised if something pops up that uses a different codon table.
True, but there could equally well be a patch of space a couple of lightyears to the left of pluto where the laws of gravity don’t apply. That doesn’t mean they are somehow flawed.
Likewise every single measurement we’ve ever taken from simple observation at a macro level, to analysis at the molecular level, strongly supports the hypothesis that all life does in fact share a common ancestor. True, some measurement might come in tomorrow that doesn’t support it, but until it does that is about as “proven” as you can get in science.
Nobody would suggest that these are not related to current life. They are descended from the same ancestor of all animals that we are. There may be large groups that left no descendants, but that’s entirely different.
Due to chemical similarities, it seems very unlikely that the current living creatures (including animals, plants, and other kingdoms) must have a similar ancestor. It is very possible that life arose multiple times. It could be very possible that life arose, the climate of the Earth killed all life, and life arose again.
Looking at a parallel situation, there were once a half dozen humanoid creatures running around until about 60,000 years ago when modern man began to dominate. The last non-modern hominoid was probably Neanderthal which was wiped out about 20,000 to 30,000 years ago. However, today all humans came from a single ancestor.
So, it is possible that once there were multiple trees of lives, but once some form of life got some advantage, it simply out competed all other types of life forms out there. This advantage could have been using DNA/RNA for information storage or the creation of proteins. Once that happened, all other creatures went from living to lunch.
We have not found a single creature anywhere in the world that possesses a different chemical processes that could point to another life ancestry.
Gosh, I guess this is where I get to flaunt my ignorance, but is it impossible that life could have begun more than once yet ending up with the same DNA system?
I’ve read about abiogenesis before but don’t retain the specifics. But don’t most people agree that abiogenesis resulted from the interaction of chemicals present on the earth? Could those chemicals not have interacted the same way more than once? Or, when abiogenesis occurred, could it have resulted in more than one similar being?
(My daughter - the MCB major in college - would be so embarrassed for me!)
As for stats, it’s kind of hard to visualize, with what we hear on TV and such, but statistics do NOT tell you what will happen. All they tell you is what is LIKELY to happen.
For instance if I take a two dice and throw them 10 times and each time the number 9 comes up, you would all be suspicious right? But statistics don’t tell us for the number 9 to come up ten times in a row is impossible. All it says is it is unlikely that would happen. But there’s nothing to say it couldn’t happen.
If that happened to me, I’d check the dice or the thrower to make sure they are both clean. But remember just because something isn’t LIKELY to happen doens’t mean it CAN’T happen.
Also when you deal with odds, think of the total. If I gave you odds of a billion to one, those are bad odds. But if you look at the population of the Earth, which is +6.5 BILLION people, odds of a billion to one say at least 6 people would fit that.
The number of cells in a human body has been estimated up to 50 trillion. That dilutes the odds so much, that it makes a improbable thing seem possible, just by sheer numbers.
Also look at flight. The ability to truly fly has devloped in mammals, insects, birds and pterasaurs. And none of them are closely related. (I didn’t say they weren’t related at all,just not closely related)
I think the idea is that DNA-based life (or even just bare DNA) is so complicated that it could not arise on its own. There are various pathways that have been proposed, usually using some sort of simpler replicating matter (such as clay or crystals) as scaffolding, replicating and competing in a simple manner until such time as they evolve the ability to synthesize DNA or even just collect stray amino acids and put them together. Once the DNA begins to replicate, the scaffolding is no longer needed and is lost. Needless to say, if it had happened any other way, chances are we wouldn’t have DNA-based life, or it might be wildly different in basic, fundamental ways.
Abiogenesis could have happened more than once, but all living things that we’ve examined have a common ancestor. If you look at the genetic code, it becomes pretty obvious where different species branched off from each other.
The interesting thing we’re learning now though is that there was a lot more inter-species genetic cross over than we originally thought and not just in microbes, due to things like viruses. So evolution isn’t perfectly linear.
Well, more than that, the connection between codons and amino acids appears to be totally random, other than things like third-position wobble. That is, there’s no particular reason why UUU should code for phenylalanine while AGU codes for serine. In fact, you can mess with tRNA genes and swap these things around artificially and they still work just fine. As far as we know, the specific code that all life uses is one of those “frozen accidents” that is one of zillions of equally good possibilities. The odds against it all happening again, and still maintaining the same code, are astronomical.
Just got off the phone with my kid. (Off topic - all you college kids out there, you probably don’t realize exactly how much your old parents appreciate it when you give the a call just to shoot the breeze!)
She said she isn’t studying evolution specifically right now, but in her MCB class they were discussing the theory of the “web of life” - in which a limited number of primative replicating cells arose after which they engaged in horizontal gene transfer. As opposed to the traditional “tree of life,” which only provides for vertical gene transfer.
(Hope I’m presenting that correctly. She really has to work hard to dumb things down sufficiently for her old man to grasp.)
So could you possibly say that although all living things share a common ancestor, that common ancestor may have resulted from different creatures? Sort of like an hourglass?
Biology classes were a long time ago but I thought scientific consensus was that flight evolved separately several times. The wing on a bumblebee, for instance, has very little to do with the wing on a seagull.
Thanks for the links, that’s very persuasive. I’ll certainly not be surprised if it turns out that there is actually only one surviving strain of life – and it seems as if that is indeed to be expected. Perhaps I’m just a bit influenced by the wow-factor the discovery of a fundamentally different kind of life would carry, and the implications it would have for the likelihood of life arising at all.
Just in the course of reading up on stuff inspired by this thread, I came across claims that the coding isn’t totally random, but rather works the way it does to minimize errors, with similar codons coding for similar (or even identical) amino acids to that end, so that some random letter change only has a limited effect (here’s a related abstract) – certainly sounds sensible to me, viewed from an evolutionary vantage point.
You’re absolutely right. It’s accepted as fact that mitochondria are the descendants of intracellular symbionts (chloroplasts are too), which were themselves descended from free-living organisms. These symbionts were bacteria, not some kind of life unrelated to everything else. We even know which group of bacteria they’re most closely related to.
There’s plenty of evidence that there’s been a fair amount of horizontal transfer (also known as lateral transfer). It’s not limited to the early days of life, but an ongoing process. One manifestation that’s unfortunate for us involves antibiotic resistance; bacteria can acquire resistance to antibiotics through horizontal transfer of genes that confer resistance. Horizontal transfer tends to be most common among microorganisms (it’s easy to imagine that physical transfer from one bacterium to another is more likely than physical transfer from, e.g., one rodent to another.
That sort of horizontal transfer, though, occurs between organisms that share a common ancestor. Indeed it only works because the donor and recipient are both based on DNA and, for protein-coding genes, the same, or nearly the same, genetic code, which entails having the same set of amino acids.
That’s not to say that multiple things couldn’t have come together very early on as part of the development of life as we know it (though I don’t think that’s what your daughter was talking about). But the result would still be a common ancestor for us all.
Not quite - abiogenesis may have happened a billion times, but gone extinct in 999,999,999 of those cases.
It’s a nitpick, but a valid one. Our reasons for saying that living things here descend from a common ancestor do not inevitably lead to the conclusion that abiogenesis is a rare thing (it may or may not be - but common descent does not conclude one way or the other).
