While I fully accept the Theory of Evolution, I do find it astonishing that there is apparently only 1 Tree of Life on this planet.
Did life really start from scratch on only 1 occasion, relatively early in the Earth’s history, and never again since?
If the Earth is in this so-called “Goldilocks Zone”, where it is not too hot and not too cold, where there is plenty of water, where conditions are so perfect for life to exist, it seems extraordinary that life has not started on many occasions over the past few billion years. The Earth clearly is a perfect place for life as we know it to flourish once it gets started.
I wondered if anyone with a good technical knowledge of biology or chemistry had any ideas on whether or not it would be difficult for a 2nd Tree of Life to get started on this planet, even if it had exactly the same DNA basis as the 1st Tree of Life. Or would it be indistinguishable from the 1st Tree of Life (I.E. could there already be more than 1 Tree of Life but it would be impossible to tell)?
If there really is only 1 Tree of Life and no technical reason why other Trees of Life couldn’t easily flourish once they got started, my conclusion would be that it must have been in the region of a trillions-to-one chance that life ever got started on this planet (otherwise it would have happened many times), and therefore even if there are trillions other planets similar to the Earth, the chance of there being any life on any of them is virtually zero.
There was a hypothesis proposed recently that there are living things from another tree of life kicking around but we don't find them because we're not looking for them in the right way. I tend to think not, but, hey, the thought is out there.
The problem for any new starting life is that it has to contend with existing life that can, with the greatest of ease, just eat it. The first replicators were fragile, ineffectual things ill-prepared to contend with the streamlined, highly evolved living machines of today - or even 100,000 years after the first replicator.
I am no expert, but I think it all boils down to the fact that life alters its environment. Eating other organisms is only one way of doing so; for instance, the atmosphere where the first organisms developed was quite different from how it is today. There was much less oxygen in the air when the first organisms developed, and when some managed to produce oxygen as a byproduct they caused a massive extinction for those that could not cope. This eventually paved the way for the much more active style of an aerobic metabolism, but another "Tree of Life" might need to develop in an anaerobic environment.
That is, I suspect, why we won't find something like a mouse from a different tree. The transition to an oxygen-rich atmosphere was fast enough to cause mass extinctions but with some organisms surviving and adapting the first time, but a tree that developed in an isolated environment only to be exposed after the transition would just be wiped out.
Hi Tuffers, Indeed, finding life from a different tree would be phenomenal. As it stands, all life that we know of and have investigated thoroughly originates from a single tree. Everything uses the same set of codons (with minor modifications), hence everything is built using the same "language", and can be said to be derived from the same pool of ancestors.
There are a number of possible alternative biochemistries that may have existed on the early earth, including alternative genetic materials (see the PNA- peptide nucleic acid- world, or TNA- threose nucleic acid- world), alternatives to the A, U/T, G, and C nucleobases, and alternatives to phosphates (arsenate- see Wolfe-Simon et al., International Journal of Astrobiology, 2009). However, our current biochemistry seems to be the most robust, hence evolutionary pressure would have selected us.
That's not to say that other biochemistries don't exist out there. Some "shadow life", like NosyNed highlights, would be hard to detect. Those forms of shadow life may use alternative handedness to their biomolecules, may use a different set of codons, or may even go more weird than that. Unfortunately, most of biological equipment is practical in nature, and hence tailored to understanding "typical" biochemistry, so searches for alternative life are quite limited.
I asked this question on another thread very recently lol
I comprehend the thing that existing life prohibits another life to develop, but I also always wondered why, even in the beginning, only one tree of life developped. I mean, there were thousands of lakes with the requirements for life to start, and the ocean is freakin' big so it could have developped wo or three times in there.
One way to think of it is through the good ol' evolutionary framework. It's possible (I'd even dare to say probable, acknowledging my complete lack of study in the subject) that there were many abiogenesis occurances on the early earth. Perhaps the first few were wiped out by a new asteroid impact, and more had to start. Maybe some that started were in a pool of water that evaporated and they dried out and died. Ultimately, only a few trees would get started enough to really be able to adapt, or be in stable enough environments that they didn't have as much anti-life forces working on them.
From there, it comes down to competition. At furst, each tree could grow as it pleased. Some probably died out, maybe some new ones sprouted, but eventually, they would come into conflict with each other. Competition for resources, predation, even on the microscopic level, would tend to favor the more adaptible trees.
Ultimately, one tree dominated over the others, and resulted in all the life we see today. It's possible there are a few scraggly trees hanging out on the fringes, clinging tenaciously to life under the shade of the massive tree we belong to. It's also possible that there are known organisms that aren't part of our tree and we just don't recognize it. Are viruses maybe a holdover from another tree that has adapted itself to us? One problem would be that any coadaptation between trees would entangle them to such a degree that sorting out if any particular branch is from one tree or from another one may be difficult, and since we can't necessarily follow the branch back to the trunk because of the limitations of the fossil record, especially with respect to single-celled, or microscopic multi-celled life, we may never know.
I was thinking on exactly the same lines as you - I understand why it might be hard for new life to get off the ground today, but why didn't several trees of life get started in the first place?
It seems that may have happened but it may be a case of our tree having the best characteristics to adapt and survive over any other trees that may have started. In that case, it may be more likely than I thought that life could also have started on other planets that may be similar to the Earth. Let's all pray for that!
Eukarya involves a merging of Archaea and Bacteria. As is entirely obvious from the genetic of DNA transcription and replication, as well as the endosymbiotic origins of mitochondria. Whether they evolved from Archaea, or from a merging of an archaeon and a bacterium (the fusion hypothesis) is not yet determined, although I favour the latter.
Archaea and Bacteria too share common ancestry as is, again, obvious from their systems of DNA transition and replication (in particular there is a 14 nucleotide sequence in rRNA conserved across all life on earth). And, here, in fact things are more complicated because there is a certain amount of horizontal gene transfer between the two domains.
As Mr Jack says, these three domains are still part of one tree of life. The biochemical similarities (RNA sequences, codon languange) between these domains are such that there's very little chance that these arose separately.
One idea I heard last year is that the three domains arose from a pool of common ancestors (see http://www.pnas.org/content/102/12/4442 - note that this is not the best reference, but the conference where I heard this doesn't publish abstracts. The second author was the one who stated this idea). This idea varies somewhat from the fusion hypothesis in that eukaryotes arose about the same time as archea and bacteria, and suggests that mitochondria were absorbed shortly thereafter. Regardless, all three domains have one ancestral lineage.