How Did We Get Here? The Ascent of Life (Numbers) – by Julian

Well, talking of numbers, bacteria have it pretty well sown up in terms of numbers. A tiny scoop of soil contains 40 million bacterial cells. There are probably 5,000,000,000,000,000,000,000,000,000,000 bacteria on the earth. There are only about 9,000 species of bacteria that are named, but estimates vary from 10 million to 1000 million different species in total. Its just that no one has had the time or energy to name and describe them all. Also some bacteria are difficult to culture in the lab, so its typically the cooperative ones, or the killers, that get the attention – so nothing new there then. Bacteria are all members of the prokaryotes, mostly distinguished by not having a nucleus in their cells, their DNA hangs loose, so to speak. We eukaryotes (animals, plants, fungi, and a ragtag collection of single celled organisms called protoists) are more conservative, we like our DNA packaged up in a nucleus, neat and tidy.

But bacteria are so small that it was only with the invention of the microscope that we even had any direct evidence of them at all. So – why are we here, when bacteria are so successful? Why bother to evolve at all?

prokaryotepn3

Well, in the last little piece I keyboarded, we saw that bacteria have a problem with size. Now I know size is not suppose to be important, but it can be if you are not careful. The largest bacteria ever discovered are getting close to 0.5 mm, which is just visible to the naked eye. These are very atypical though. As I talked about last time, the surface area to volume is a key issue for anything that eats and makes energy through its surface. These very large bacteria typically don’t actually have a very large active volume.

Thiomargarita namibiensis for instance, one of those big boy bacteria, has a huge vacuole inside it. This is basically just a store of stuff it is keeping for a rainy day. It doesn’t therefore have a large active volume, more like a thin layer doing stuff, and a big garage full of useful stuff it can raid when it needs. Others get around the issue by changing into long thin shapes, or by having folded membranes to try and offset the problems. All very clever, but it doesn’t make any fundamental leaps in improvement.

actin

What we needed was a paradigm shift. That needed a few things to have evolved first, before it could actually come about. The first step was probably a cytoskeleton. A cytoskeleton does for a cell what our skeleton does for the body. It gives it shape and structure but is inside. The original alternative, the cell wall, was more like the exoskeleton that insects and others use to keep their shape. We’re about to loose the cell wall that most bacteria have, and to stop them collapsing in a heap, they need an internal skeleton. This is mostly made from a rope like protein call actin, together with some securing bolt-like proteins, that hold them in the shape they want to be. A cytoskeleton use to be thought of as purely a eukaryotic accessory, but has been found in a few bacteria too more recently. So obviously some of them managed to make one.

Why develop a cytoskeleton at all if the cell wall is so useful? Well one answer is in things like penicillin. Penicillin is a very effective antibiotic because it interferes with the building of bacterial cell walls, and so they tend to rupture and break. It doesn’t worry us, as we have no cell walls to make or break, so it doesn’t interfere with any of our key process (if you ignore allergies). Lots of things produce anti-bacterial wall substances, its part of the arms race. If you’re a bacteria feasting on some supply of food, and you can pump out one of these substances, then there is all the more for you, as long as it doesn’t affect you. So its possible that one species of bacteria happened along the mutation that gave them a cytoskeleton, and this was another way of avoiding the attacks of their competitors. They could laugh in the face of such attacks.

eukaryotelv0

Now once you don’t need the cell wall, new avenues open up for you. The cytoskeleton is like a series of ropes holding things together. If you shorten one you change the shape of the cell. With a bit of practice you can use this ability to stretch out, contract, or even make funny shadow puppets. One nice feature of this is that instead of sucking in dissolved food, you can extend yourself around food particles and ingest them in one gulp. It doesn’t matter how many competitors there are around now, as you have the food inside ready to digest at your leisure. Of course you can’t do this with a thick rigid cell wall, but you can with a cytoskeleton.

So there we are with a bright shiny new ability, going around ingesting food in lumps and digesting it at our leisure. Cell walls still have their uses of course, they’re tough for one thing and can fend off all sorts of nasty things, so most bacteria stuck with the tried and tested mechanism. The new wall-less organism probably got beaten up on a number of occasions, and were probably humiliated in public places too. One day though, it managed to swallow a working bacteria, and couldn’t digest it. Furthermore, the swallowed bacteria continued to live, sucking in nutrients from the cell attempting to digest it. It is quite possible this happened at the time of rising oxygen levels, and from this a kind of symbiosis happened. The swallower was having a problem with the highly reactive oxygen molecules wreaking havoc with its cellular processes, but the swallowee was already well versed in the handling of this and so consumed it. So the swallowee was getting protection and a source of nutrients, the swallower was finding a way to deal with the increasing levels of oxygen.

Gradually over time the bond got stronger, the swallowee began to discard a lot of DNA, as it could get most of what it needed directly – all the materials for making a new version of itself were there for the taking. Say a defect occurred in the DNA for making cell membrane proteins, no worries just take the ones the host cell has already made. Meanwhile the swallower started to take advantage of the swallowee, by punching holes in its membrane to get hold of energy molecules that it was making directly. In this way the power house of eukaryotic cells, the mitochondria probably came about. Tracing back through genetic history, it seems likely that Rickettsia prowazekii is a modern descendant of the original mitochondrian. Today, our mitochondria still have their own DNA, though now it is pared down to the bare minimum, and they still divide and reproduce independently.

Anyway, these new organelles (as they are called), as well as a giving a way of dealing with excess oxygen, also had another benefit. Suddenly it was not so limited by the volume/surface ratio. If you need more power, then grow more mitochondria. Lots of membranes available then to pump protons across now, the restrictions are lifted. You can’t go completely wild though, as there are still things that are restricted by size. We don’t see single cells much bigger than about 1cm even today, and even those have to play all sorts of structural tricks to get so big. A typical modern cell may have hundreds of mitochondria inside, making all the energy they require. Divide and conquer so to speak, but making sure you are the sole beneficiary. Energy requirements – sorted!

mitochondrion2

So now our cell can grow much bigger and still have plenty of power. It can ingest food in large lumps and so doesn’t have to compete by breeding as fast as possible. It can be much more leisurely about breeding. This in turn means that DNA is not so much of a limiting factor. It can start to accumulate junk in its genome. Duplicate copies, broken versions and so on. Sure it costs a bit more to replicate, but its not a life or death issue any more.

Its not just energy factories that were swallowed. Chloroplasts, those organelles that plants and some protozoa use to turn sunlight into sugar seem to have suffered a similar fate. They also have their own DNA and replicate themselves. They would be much more important if we were a plant based species, but as we’re not, I’ll skate over them. Another theory, without quite so much support is that the nucleus is also an ingested item. Anyway, these cells with all their new found machines were one of the steps that led to us. Although these new cells, the eukaryotes are much bigger than bacteria, most of them are smaller than the naked eye can see, and stay that way.

So how did they get bigger?

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