proto-biology questions

Hi there,

Although I have some specific questions, there aren’t definitive answers for them probably, so I’m putting this in MPSIMS.

I’m watching “The Origins of Life,” on Hulu. The photography is stunning. And already, 15 or 20 minutes into it, I am stricken by the fact that we know a whole lot of the “whats,” but (necessarily, I guess) hardly any of the “hows.” The narration says things like, at this point, single-celled organisms began to join together for mutual benefit. Or, x years ago, organisms started reproducing sexually. But I have all these questions.

The example they showed of symbiosis-which-led-to-a-single-organism was coral. As I understand it, they were suggesting that a similar process may have led to the formation of new organisms, for instance, I have heard it speculated that mitochondria are likely to have originally been bacteria that entered into symbiosis with other cells. I know it can’t be proven, but are we pretty sure that that’s what happened? Are there other examples where we know for sure that one organism incorporated another, different organism, and produced a new creature altogether?

And about sexual reproduction – it’s hard to picture the steps leading up to this development, but it couldn’t have happened all at once, could it? Did the immediate predecessors to the first sexual reproducers just divide themselves, and the next generation do something new? What kinds of things could have precipitated such a development; I mean, how did organisms prepare to begin reproducing differently?

I have a good friend who insists that all evolution is the direct result of natural selection, pointing to the Peppered moth as an example. But I wonder if it is possible that a given population might sometimes respond to changes in their environment with an innovation that is not merely the result of a random mutation being better equipped for survival. Is it currently believed that sexual reproduction was introduced as a mutation?

(I don’t mean to tick off any actual scientists with foolish questions or ideas. This is just one of those situations where I find that my curiosity far outstrips my education.)

I’m going to move this over to GQ, where there are a lot of knowledgeable – and patient – biologists who have always been happy to answer my periodic questions about evolution and suchlike.

twickster, MPSIMS moderator

It’s not really that a new creature was produced. It would have just produced a symbiotic relationship and then presumably the two adapted to the symbiotic relationship until they were unable to survive on their own. Lichen are pretty good examples of how this may have worked; the two organisms (the fungus and the algae) are still separable, but they don’t survive very well on their own.

I don’t think anyone knows for certain. However, bacteria can exchange genetic material. It isn’t sexual reproduction exactly, but it does seem like something in between. Many species (plants being a good example) are able to both reproduce sexually and by cloning, so an early form of sexual reproduction that was highly unreliable wouldn’t necessarily kill off the species that had it.

Unless you want to make it a theological question, you’d have to go with your friend’s explanation. There really are not any natural processes for the rapid response like you’re talking about - even epigenetics can only influence which genes are turned on or off, not the structure of the genes themselves.

Pretty much by definition, no. The only way we could be certain that this was the case is if we could separate the two organisms and they could exist independently. We have plenty of examples of that type with Dracoi and your OP noting the best examples. But by definition such examples have not “produced a new creature altogether”. IOW any example where we can prove that the creatures are separate organisms by definition can’t have formed a new creature altogether.

The first thing to realise is that most organisms that reproduce sexually even today are perfectly capable of reproducing indefinitely asexually. You may get a hundred generations of asexual reproduction and the a single generation of sexually produced offspring, or you might get an alternation of sexually and asexually reproduced offspring. But the important point is that obligate sexual reproduction is the exception, not the rule. IOW we animals (and seed plants) are freaks, and very atypical.

So the predecessors of the first reproducer did indeed just divide themselves, as did all their immediate descendants. Sexual reproduction was a reproductive option. It wasn’t mandatory or even common.

As for how they did it. The evidence of that still exists today. Many organisms still undergo what we call plasmogamy, where the cell membranes merge and the cell contents of the two cells become mixed. That’s not necessarily being done for sexual reproduction, it’s often just a cheap and easy way to double the available genetic material. It benefits both partners because they get to exploit useful genes that they don’t carry themselves.With the cell contents in these fused organisms in such close contact they also become affected by the same environmental conditions and hormones, and inevitably nuclear multiplication will become synchronised.

At that stage true sexual reproduction seems to have evolved as a means of checking for viruses and gross mutations. Each nucleus aligned its chromosomes with the chromosomes of the other nucleus and ran a comparison. Any gross differences were likely due to viral infection or mutations, and that chromosome or chromosomal section was then culled and replaced with a duplicate from the uninfected partner.

And while it seems to have evolved initially as an error checking mechanisms, this process also gave the advantage of increasing genetic diversity by giving each nucleus direct access to novel genetic material, rather than being dependent on plasmogamy to achieve the same end. And so sexual reporduction became and end in its own right.

It certainly is. Such processes have been well confirmed in the last decade or so.

Depends what you mean by “ introduced as a mutation”. It is certainly the end result of a series of mutations, but there wasn’t any single mutation that suddenly produced sexual reproduction. Like all complex traits, it evolved as a series of small steps, it didn;t come into being as a result of a single genetic variation in one generation.

Chloroplasts, as found in plants & algae today

I’m not sure I catch your meaning here. Could you give an example, perhaps?

PDF here.. You can get plenty more info by doing a Google scholar search on epigentics.

Ah, thanks. I wasn’t sure whether the recent discoveries in epigenetics, which I’m at best tangentially aware of, were actually what you were speaking of.

Thank you, Blake. When dracoi first mentioned epigenetics, even though he thought that wasn’t what I meant, I had to go look it up first; and then I went, “Eureka!” The concept pretty much explains the kind of thing I was talking about. Which triggered the realization that evolution is when the DNA has actually changed; I hadn’t realized that the kinds of changes I was thinking of could be simply the expression of previously existing, latent genes. Makes perfect sense. (except, *how did the genes know how to plan for every constituency? * I think that’s amazing!)

When I referred to a whole new organism from two, I was pretty much thinking of mitochondria, and whatever they bonded with that eventually became us. If they did originate as bacteria, that is also pretty cool to think about.

I was not aware of the process of plasmogeny. I can picture the leap from mingling your genetic material in physical contact to releasing that material into your immediate surroundings; the whole thing is easier to understand.

I envy biologists; although I know their work can involve a lot of drudgery and waiting, still it must be a fascinating field. So many things yet to figure out!

Not even all animals, at that. There are quite a lot of animals (some insects, some fish, some reptiles) that reproduce parthenogenetically, either exclusively or in addition to sexual reproduction.

And almost all plants can reproduce asexually by vegetative means. Now, whether or not this happens all that often in the wild is another question, but it’s certainly common for humans to take advantage of that ability.

A) I think you mean “contingency.”

B) There is no planning. This is because there is no knowing. Either the latencies are there or they aren’t. If they aren’t, the line dies out.

There’s no planning going on. Certain genes are confer a survival advantage under certain conditions, and when those conditions no longer exist they are switched off, but they aren’t removed. When the descendants of the organism encounters those conditions again the genes are switched back on and the trait is expressed. That’s a bit if simplification, but it’s close enough.

To give a human analogy, it would be like humans moving out of the tropics and the genes for long-limbs and dark skin being switched off, but not actually being altered via evolution. So that when Europeans move back to tropical Africa they immediately start giving birth to long-limbed, dark skinned children. It wouldn’t require any planning or active knowledge. All that’s required is that the body respond to high temperatures and high UV light levels by re-activating the ancestral genes that evolved to cope with those particular stresses.

So the genes haven’t planned for every contingency. They have only “planned” for those stresses that the ancestral organism had to cope with. All that has happened is that genes that evolved to cope with stresses in past generations have been retained, though they are no longer expressed. Expose the organism to the same stress and the genes get switched back on, which is much faster than having to re-evolve the same genes.

The really interesting part for me is how the organism knows which genes to re-activate in response to which stresses.

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While these are certainly examples of asexual reproduction, they aren’t really relevant to the OP because they are secondarily derived from sexual reproduction. IOW the ancestral condition was sexual reproduction, and that has evolved into novel forms of asexual reproduction. So it doesn’t really shed any light on what life was like before sexual reproduction.

Your good friend is wrong. There are other evolutionary mechanisms, i.e. processes which result in changed allele frequencies in a population. Natural selection may not even be the dominant mechanism.

Shmendrik: I am all ears! (figuratively speaking. I’m not actually expressing those genes right now.) If you or someone else has a moment, please elaborate on the abovementioned processes.

Meanwhile, I still think that at some point, there is something akin to, I don’t know, maybe “cellular wisdom” going on.

Twickster: You’re right. I shouldn’t post before I’ve had my coffee.

All of this stuff that seems so amazing to us is simply the result of proteins interacting with signal molecules and other proteins. There’s nothing mystical or magical about it. It’s just the simple evolution of signal transduction pathways. They’re generally well understood in general terms, if not always in the exact specifics. Genes are turned on and off by proteins, which are activated and deactivated by other proteins, etc, etc, until you find the proteins that interact with the external environment.