What are the reasons for believing that life on earth has a single source?

Apes, men, common ancestor, yadda yadda.

Nice theory there. I won’t comment on it because I know next to nothing about evolution and anthropology.

But what about all life having a shared primordial ancestor? I read ages ago that this was the currently accepted theory. What are the reasons for beleiving it to be true?

Why not lots of different begginings for different species with a similar chemical catalyst?

It depends of course on what you mean by a single source; it seems likely that similar chemical processes would be happening in all sorts of places, but even the initial transition to ‘living organism’ must surely have happened to a population, rather than an individual (population in the sense of a large pool of chemicals, rather than a single self-replicating protein floating around alone in a huge sterile environment)

One of the biggest reasons is the fact that DNA can be made in two forms: one with a right-handed twist, and one with a left. However, all life on Earth that we know of has DNA with the right-handed twist.

To add to Q.E.D.'s comment, all amino acids come in a left- and a right-hand form. Both are equally likely to form, but all life uses uniformly only one handedness for all 20 amino acids.

Oh, and then there’s the fact that all life uses the same 20 amino acids, when there are god-knows-how-many different ones that could be constructed with different functional groups.

And the fact that all life uses nearly the same genetic code: There are 20 amino acids which must be coded for by four nucleotide bases. Therefore, life needs to use at least three bases to code for each amino acid. For example, AUG codes for the amino acid methionine. There is no inherent chemical or physical reason to favor any code over any other, yet, with a few very minor exceptions, every life form on earth uses the same code. Very strong evidence that all life arose from a common source.

Finally, all life has very similar cellular machinery. Common to all life is the cell, ribosomes, rRNA, tRNA, DNA polymerase, and about a million other molecular machines. Also, all eukaryotic life has very similar mitochondria, nuclei, centromeres, etc. The chance that multiple-origin life would all come up with the same chemical machinery to accomplish all these tasks is incredibly remote.

bryanmcc hit on all the major evidential points here.

It is certainly possible - perhaps even probable - that very early on in the history of life, there were multiple origins within different populations; e.g. life may have “arisen” more than once. Maybe one line used a different handedness in nucleic acids, or maybe even different nucleic acids or different genetic mechanism altogether. Maybe there was a competing group with a very different genetic code and set of amino acids. But there are no remnants of any such lines today. So this means that any other lines, if they did ever arise, have died out in competition with our own. So it is correct to say that all extant life is single-sourced.

What is most remarkable about this, and what makes it so compelling, is that there are NO exceptions known. We have discovered millions and millions of kinds of life as different as humans from sequoias from lichen from bacteria that eat rust and never see the sun – and every last one of them has the SAME kind of DNA, proteins, and other macromolecules! The correlation is remarkable. The fact that no exceptions have been found makes it extremely likely that all life has a single source.

[geek]This is one of the real cool things about science – here is an excellent example of the power of falsification and the ease with which science can accept challenges to its own theories. If anyone ever found an organism that had a totally different genetic code or set of amino acids, or indeed a different genetic or sructural/enzymatic molecule set altogether, Biology would not fall apart. Indeed, it would energize research into molecular biology.[/geek]

-mok

*As bryanmcc also pointed out, there are minor differences found from place to place in the genetic code, and even in the amino acid set. But these differences are extremely minor, and are not differences in type. They all fit into the same universal molecular framework.

All organisms, excepting RNA viruses, store the information needed to make essential proteins in their DNA. The genetic code is universally comprised of the 4 bases adenine (A), guanine (G), cytosine©, and thymine(T). The amino acid sequence of each protein is coded for by triplets of these sequences called codons. With three bases per codon, and four choices for each base, there are 4 X 4 X 4 = 64 possible codons that could specify any particular amino acid. There seems to be little adaptive advantage of one coding scheme over another, although there is some debate over the fitness of various coding schemes, so there are literally billions of different schemes that independently evolved life forms could have developed. With very minor exceptions, all life forms share the same codon to amino acid translation table. Coincidence ?

I don’t buy it.

It is possible that conditions specific to Earth accounts for the similarity of DNA codes for different creatures.

Having something in common with another species does not necessarily imply that we are related. It might be that we simply underwent identical evolutionary changes due to the similarity of our circumstances.

But for ALL those different factors to have come about exactly the same is staggeringly improbable, even under similar conditions.

Meta-Gumble, I’ve had several dozen biology and other science courses, and I can come up with no reason, either chemical, physical, or environmental, why the sequence of DNA bases Adenosine-Cytidine-Thymidine should code for the amino acid Threonine (for example – any of the other nucleotide-amino acid combos would work the same). And I’m not claiming that I have any particular expertise – I am fairly in touch with the literature, and I have never seen any legitimate researcher claim that the particular code is an adaptive response to environmental selection, or is as it is for any other reason than random chance.

It seems to be entirely arbitrary – there are no chemical properties of particular nucleotides that attract particular bases (the translation of the code is handled by machinery that keeps the molecules separate); there is no physical information in the code that pertains to particular amino acids (the nucleotides aren’t a particular size or shape that imitates the amino acids); and there are no conceivable environmental factors that could select one code of over the other (threonine coded by ACT rather than by TTA or CAT does not help bacteria reproduce or gazelles escape from attacking lions). Therefore, the only reasonable conclusion is that the identity of the genetic code across the tree of life is historic. The chance that it would be coincident between even two unrelated branches is truly astronomic – something like 20^64 to 1 against.

Let me clarify my above rambling. Having something in common with another species does not necessarily imply relation if that something is useful. For example, birds, bats, insects, and pterosaurs all have (or had) wings. However, this doesn’t necessarily imply that they are closely related because wings are useful, and similar selection acting on each group could have caused them to evolve them independently. And looking at the skeletal (or lack thereof in insects) structure of the wings clearly indicates that this is what happened.

However, if something is arbitrary, it can be a very good indicator of close relatedness because of the very low chance of evolving the same way if there is no selection. The particular genetic code seems to fall in this category – it would have worked just as well with any other code. The fact that all life shares this same arbitrary code is therefore strong evidence of a common origin.

It is not just DNA. It is DNA, and proteins, and cell organelles such as Mitochondria, which carry its own genetic material.

Occam’s razor is a sharp and useful tool for cutting out needlessly complex theories. Separate but identical evolution requires too many “coincidences.”

As for why it would have happened this way, it takes a long time to get life from “scratch”, millions of years at least. But once you have life, it doesn’t take it very long at all to spread to cover the entire planet. Once one sort of life gets a head start, any more independent lifeforms wouldn’t stand a chance. The moment a new, independent cell formed, it’d get eaten.

In the same way that it’s possible that conditions specific to Europe account for the fact that French and Spanish are so much alike. But I’m going to stick with the “they both descended from a common ancestor” theory over the “they spontaneously arose in isolation and just happened to be remarkably similar” theory.

But you have to admit that if it happened once (a chemical reaction, fulfilling the [rather loose] definition of life) under certain circumstances, it could quite easily happen again under similar circumstances.

It is only speculation whatever you choose to beleive, but I don’t think there is any more reason to beleive that all species evolved from a shared primordial ancestor.

I see no reason to suggest that a heterogenous development of life is unlikely, and brianmcc’s; “something like 20^64 to 1 against.” seems to be arbitrary. Could you show your working for this calculation please?

You seem to have ignored my post.

Let me give you an analogy.

If, at a murder scene, detectives find DNA, finger prints, and hair samples that match a particular person, is it beyond reasonable doubt that that person comitted the murder instead of Gok the Space Traveller from Beta Lyrae?

Yes, given another primordial earth, life would almost certainly arise again. What everyone is saying is that the likelihood that it would have exactly the same molecular machinery is staggeringly low.

As to my calculation: There are 20 different amino acids and 64 different triplet nucleotide codings, and we’ll assume that our hypothetical second lineage of life forms uses the same set (though there doesn’t seem to be any reason why they would have to). Therefore, we need to figure out how many different combinations there are for coding 20 items with a set of 64 items. I don’t have the mathematical skill in permutations, but it’s something like 20^64 different ways you can create a correspondence between the two groups. I’ll invite someone with the ability to help out at this point and produce an actual equation to determine the exact number of different combinations.

If you want to convince yourself that this is a huge number, get twenty dishes and 64 numbered poker chips and figure out how many uniquely different ways you can put them in the bowls, so that every bowl has at least one chip. If, after the first few million years of putting different combinations of poker chips in bowls, you still aren’t convinced that this is a large number, come back and we can talk some more.

Another clarification (I really need to learn to say what I mean the first time): Codon 1 can code for any of 20 amino acids. Codon 2 can also code for any of 20 amino acids, including the one that codon 1 coded for. So, with two codons, there are 20 x 20 = 20^2 = 400 possible unique genetic codes. There are in fact 64 different codons, so you can create 20 x 20 x 20 … x 20 = 20^64 = 1.845 x 10^83 = two hundred billion trillion trillion trillion trillion trillion trillion different possible genetic codes. This number is about a thousand times greater than the estimated number of particles in the universe. This calculation has neglected the fact that, in a genetic code, every amino acid needs to be coded for at least once, but you can clearly see that there are a huge number of codes possible, and only one present.

Then there’s the so-called rare amino acids. There are a lot of them, and there’s plenty of room in the codon translation table (64 slots vs 20 common amino acids) to include them in the makeup of independently evolved life forms. Yet no lifeform has a codon for the likes of ornithine or hydroxyproline.
http://www.biologie.uni-hamburg.de/b-online/e20/20e.htm