How Did We Get Here? Clubbing Together (Deuteronomy) – by Julian

Deuteronomy – the second law. Well the reference is a little oblique, but we are sort of in the second era of life, the eukaryotes. That is the cells with nucleus and other internal organelles. They are not restricted by the same volume to surface area energy generation that bacteria are, or by the need to ingest food through a thick cell wall. They have their limits though. They still need to get in certain things across the cell membrane. Oxygen, water, various other things too.

acetabulariaMost cells get stuff that they need in the place it is needed by doing not much. They just wait for things to drift by and arrive in the right place. This is a surprisingly efficient way to do things it turns out, provided the distances involved are really small, say the size of the average cell. If you wait for stuff to diffuse over large distance, you can be waiting a while. Acetabularia for instance, is a single cell that grows to sizes of 1cm and upwards, pushes this technique to the limit. It has all sorts of modifications, but has a sort of internal current to move stuff around in a timely manner. That way stuff it has made can get to where it is needed without waiting for ever. Other than these few show-offs, most cells are much smaller, small enough that you can’t see them.

So we’re up against some limits when it comes to size for a single cell, and very few make the effort to get big. Instead, most decided the way to get bigger was to party. A group of like minded cells would get together and go around in a gang. Quite how this happens is the subject of some debate still. However there are some good examples of modern single celled organisms doing this. Slime molds are one good case of this. These spend most of their time as single cells, doing their own thing. However on receipt of a certain chemical signal, its like a meeting of the clans. All the single cells group together and they start to join up and they behave as one. Some of the cells become spores to form the next generation, and some act as a support network to host the spores, and pay the ultimate price for the privilege.

elephant-ear-spongeIf we move a little way up the development spectrum we come to sponges. Sponges are not very advanced anatomically speaking. They only consist of a few different types of cell, and what’s more you can mash them up into their individual cells. Given time though, they will reform just like an old rock band. So we can see that these sponge cells, although making part of a bigger organism, are really only loosely bound. Try dividing, say, a dog up into its individual cells, it is not going to have a very good outlook. Sponge cells are a little more free spirited though. What’s more, you can even take two different species of sponge, of two different colours and reduce them to their parts, then mix them up. After a few days they will reform back into their original groupings – red with red and yellow with yellow.

So what advantages are there to this multi-cellular lark? Well probably the original one was size. Yes, I know, we’re back to the whole “size is important” thing aren’t we. Well yet again it is if you are about to be eaten. Generally speaking, large things are harder to eat than small things, and things that are larger than you pose a more difficult problem. So there is safety in size, which in this case translate to safety in numbers.

However we also get another potential benefit going to multicellularity – specialisation. Sponges don’t go too far down this route, they have a small number of different specialised cells. One of their main cell types are basically digestive cells. These have a large surface area, and also beating flagella that waft water past them to hopefully get food. These cells bare a striking similarity to a type of single celled organism, the choanoflagellates. Hmm – very suspicious that, do you think that sponges have a deep dark family secret?

Other cells in the sponge are concerned with binding it together, others act as a sort of skin, and others major on gathering calcium from the water to make limestone. This helps give them structure and a sort of skeleton.

So we can see the beginnings here of something bigger being formed. Of course it is a major paradigm shift again. Where once it was look out for yourself, now you are part of something bigger, and your wishes to survive as a single cell may not agree with the wishes or needs of the organism as a whole. It sounds a little like democracy at work, and one individuals desire to do things their own way may well be in conflict with the majority. There is a common event in multicellular animals called apoptosis, where a cell voluntarily commits suicide. This can be because it detects it is malfunctioning, or because it has been told too. This is not the sort of behaviour you would see from a single celled organism. For them, death is death, and they aren’t going to give up their one chance on a whim.

At a very similar time, another powerful mechanism came about, and there is no way to skate around it – we’re talking sex. Sex is very interesting for a number of reasons (all right -stop smirking at the back there and pay attention). Why come up with sex? I mean, what’s the point (oyyy!). As Richard Dawkins said in one of his books, the more you learn about sex, the harder it gets (stop it…) to explain, and the more complicated the reasons behind it appear. In the end you end up scratching your head wondering, just, well, why??

Lets try and explore it a bit. There has to be some benefit to it, otherwise it would lose out to asexual organisms. Lets compare an organism that reproduces by sex, B, with one that doesn’t, A.

If A wants to breed, it just does it. B has to find a mate. So on that score A is going to well outperform B. While A is busy getting on with the next generation, B is composing “like to meet…. GSHO… walks in the rain…” type signals.

Then again, to preserve the breed, all A has to do is divide, and the next generation is all set. For B both a male and a female have to be made. If B gets with a likeminded B and produce a bouncing baby boy B offspring, the next generation is not assured unless they or others produce a girl. So again A is winning hands down at the game of survival.

Now we know that evolution can generate some horribly non-optimal systems on occasions, but a system of breeding that is used by nearly all the multi-celled animals has to have some redeeming qualities else it would have died out years ago.

Before considering the advantages – lets look at an alternative first. Bacteria have a sort of sex and have done for years. They call it conjugation, but we all know what they’re really doing behind the virus sheds! Bacteria keep most of their DNA in a big ring, with all the genes they might need on it, or nearly. They also have floating around little plasmids which are typically much smaller rings of DNA. Sometimes when one bacteria runs into another, they will form a little tube between them, and send a plasmid down it from one to the other. This allows them to swap bits of DNA. So the first bacteria might be resistant to methicillin for instance, and by passing this plasmid containing the methicillin resistance across, the 2nd one is now also resistant, welcome to the club, MRSA. So if one bacteria generates a useful mutation, it can spread through the population.

ti_plasmid

So – to recap, asexual beings like bacteria can reproduce any time they like, they don’t need to find a partner. They only have to produce one offspring to carry on the race. They have ways of passing around good genes that can get used straight away. So, things look pretty good for these type of organisms. In a straight race, they are going to win every time – while the opposition is out buying flowers and chocolates, they are getting on with it.

So what possible advantages could sex bring that makes it popular across the animal and plant kingdoms. Oh, and the fungi kingdom – although fungi take it to extremes with sometimes hundreds of different sexes – imagine what that’s going to cost you in flowers!

Well, the positives from sex are a little vague, but one clear one is the recombinatorial effects. You take your recipe book (DNA), split it in half, combine it with someone else’s, and you have shuffled the overall recipes. This lets you try out existing things in new combinations. Brown eyes with fair hair, long legs with big ears – the combinations are enormous. So that is one thing it brings – you evolve faster, and from things that already work, but might possibly work better or worse in combination. Long legs and big ears might be better than short legs and small ears, or might not depending on the environment.

three_cell

The other thing is getting rid of bad recipes – to continue the analogy. If you have something bad in your DNA – say a gene for weak legs, you can get rid of it. If you have two copies of the weak leg gene, then providing you don’t pick a similarly afflicted partner, your children will have only one bad leg gene – one bad one from you, one good one from your partner. Then it depends if bad legs are dominant or recessive genes. If they are recessive (and a lot of bad things can be recessive – they are the absence of something often) then all the children will have good legs. Their offspring in turn may not inherit any of these genes – its a 50-50 chance on each mating. The bad leg gene could be history in just two generations.

Bacteria reproducing asexually have a much bigger problem. If they get a bad gene by mutation it is much harder to get rid of it again. Their offspring get all of their genes, good and bad, and no choice in the matter. To get rid of a bad gene, they have to hope for a mutation to happen mostly. They do have a couple of other tricks based on their slightly odd sexual behaviour with plasmids, but they have a much bigger issue getting rid of bad mistakes.

Some organisms play both ways, so to speak. They mostly reproduce asexually, which is nice and fast, but will have sex if the opportunity presents itself. However by and large, most complex organisms, animals, plants – and oh yes, those crazy fungi, have opted for sex, and they stick with it. There are always a few species that like to break the rules of course. Bees, ants, termites – they are a little odd. Some fish go through all the motions of sex – including the physical, but actually reproduce asexually when it comes down to it – weird!

Anyway, it seems one way or another sex was pretty important in the story of how we got here. Its probably been around for some 800 million years, and shows little signs of fading.

So once we have this ability, and the multicellular plan, what happened next?

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