Unlike the genocidal story of Joshua’s conquest, this is the story of not just how a genus came to be, but how the very phyla came about – and yes, the links are becoming increasingly tenuous, but I got myself on this track and can’t see to get off! However it does end with the battering down of the last barriers to modern animals, so I guess, well, yeah, that’ll work.
If we look back in the fossil record, there appears a big explosion of forms and shapes back in the Cambrian period. That was originally about as far back as people use to think life went, which is why stuff before that was called Precambrian and considered boring and uninteresting. Nothing much happened before that, everyone knows that. It was also known as the Cryptozoic – that time when life was hidden, mainly because they couldn’t find any. However with better dating techniques and exploration in various places it is not quite so hidden as it was before. That, and people wanted to split up the 4 billion year lump of the Cryptozoic into something more manageable, meant some new terms for the eras came about.
So we have the Hadean, which covers the time from the Earth’s formation to about 3.8 billion years ago. Any stuff at all from this era is pretty rare, and rather boring as far as life is concerned, there isn’t much of it and what there is has been through the mill a bit. Ocean floor material gets recycled very quickly geologically speaking – give it a 100 million years or so and it will have got sucked back into the mantle generally. Not much use if you want to look at stuff billions of years old. The only stuff that can survive billions of years is continental material which tends to float on the crust. However continental material is subject to weathering, continental collisions, rivers, seas, glaciers, impacts, life and gigantic laser beams (ok so the last is not a huge factor, but James Bond has been responsible for a certain amount with his waging wars on hollowed out volcanoes). So stuff that is still around after 4 billion years is unlikely to be as it was formed – immaculate condition, still with its original box and instructions.
Then there is the Archean which runs from 3.8 billion years ago to 2.5 billion years ago. This is where the bacteria lived and thrived alone.
As you can see in the Geologic Clock, next we have the Proterozoic which takes us from the Archean up to the Cambrian, which starts off the Paleozoic at about 542 million years ago. So what was life doing in this time?
Well if probably had it tough, as it looks like it turned might cold around that time. Its still an area of dispute but there seems some reasonable evidence that between about 790 and 630 million years ago the earth froze fairly solid. Known as the snowball earth theory it was a time of runaway cooling. There is certainly some dispute about how much coverage there was, and whether it might have been a more slush-ball earth than something covered entirely in glaciers. The theory goes a lot of the carbon dioxide in the atmosphere got fixed into the oceans and crust. This got rid of one of the greenhouse gases that we’re all so aware of these days, and tipped the balance with not enough heat being held. As the glaciers advanced, their shiny surfaces reflected more and more heat back into outer space, so the condition got worse and worse.
So what was life doing during this big freeze? Well it may well have taken a knock or two. If the whole earth was covered in glaciers then photosynthesis would have been rather difficult. Life can survive even in the Antarctic today, although bacterial life is rather sluggish there. So it could undoubtedly have survived although mostly by hunkering down and not doing very much. Single celled organisms could also have probably survived some of this too. Several types of organism can form spores or cysts where they basically more or less dry out inside a thick shell and wait for better times. Bacteria use this technique today to avoid nasty situations, including being dosed with antibacterials. Any inhabitants of deep ocean vents, where energy from the sun is not required would have done just fine, although they have their own issues with vents not being long term stable things. It is believed that eventually volcanic activity managed to disrupt the big freeze and set things back on course.
So – somehow life got through the big freeze, and the next thing we notice is some large forms of life making their appearance. One of the first Precambrian large scale fossils, Charnia, was found in Charnwood, Leicestershire, UK. It was discovered by a school boy in a small outcrop of ancient rocks, which were already well charted by the British Geological Survey, so was well known to be of Precambrian vintage. This allowed other sites to be considered, now that Precambrian fossils were a reality, and the period came to be known as the Ediacaran period. This was named after a large number of Precambrian fossils were discovered in the Ediacara hills in Australia. Originally thought to have been basic Cambrian fossils (as everyone knows there are no fossils before the Cambrian…), they later turned out to date much earlier once analysis was carried out. There are all manner of weird creatures preserved, and oddly, they are soft bodied creatures which usually don’t get preserved in fossils.
Several things are necessary for fossilisation to occur. Firstly its much easier to fossilise something in water than on land. Its easy to get layers of sand or silt to cover things up in water, not so easy on land – but then all life was in the sea at that time anyway. Secondly, they need to be covered reasonably quickly. If not, then either predators, or bacteria will get to them and start to devour them. Oxygen poor environments are good as most things require oxygen by now. Third, they need to get covered with the right sort of stuff – fine sand or silt is best so detail can be seen. Pebbles wouldn’t be very good for creating a cast for instance. Then the site needs to lie undisturbed for long enough for the matrix to solidify, and that may take many many years.
Clearly something odd was going on around that time that allowed the preservation, and a similar thing happened in the Cambrian when the Burgess Shale got made. A landslide of bud taking a collection of stuff deep down and covering them up with silt is one of the more popular theories, which would give a good snapshot like picture of life then. This also preserved a good number of primitive species, and we find all sorts of stuff in that, including the beginnings of what we will be calling the different phyla in the future.
So what is a phyla? Well broadly speaking life is divided up into various categories, of increasingly finer divisions. At the very top are domains of life. There are three of these, Bacteria, Archea and Eukarya (we’re in that one). Below Eukarya, we have Animalia (animals – that’ll be us), Plantae (plants), Fungi (lets see …fungi!) and Protoista (single celled organisms). Then the next division down is the phyla, or phylum if you’re talking about one.
For animals there are about 30+ different phyla, and these are broad structures. Some of the these phyla only have 1 or two known residents but most have a lot. We’re in the phyla chordata – which is those which more or less have some sort of backbone or backbone like thing going on.
Some of the more familiar phyla are Mollusca (slugs, snails, but also octopi and squid), Arthropoda (insects and their ilk), Echinodermata (starfish and sea anemones), Annelida (garden worms – segmented) and Nematoda (round worms). Now quite a few of these can be traced back to the Cambrian and Precambrian, but as you might expect by looking at a thick branch of a tree, it probably didn’t have all the features it has now when it was a twig. Some of the Cambrian and Precambrian life forms look just plain weird, and it seems obvious that there were a lot of “well this seemed like a good idea at the time” types of evolution. Quite a number of species can’t be slotted into any modern day phylum, they just don’t follow any of the rules used to type them.
Of course we only get a tiny view of what was going on at the time, not much has fossilised, and you have to wonder how representative that that did get fossilised was of the mainstream stuff. Are we just seeing fossils of things that didn’t do very well, or happened to exploit a small niche that also happened to fossilise well? Oh for a time machine…
Of course there are also a number of things that are missing at this time. It is believed that Chordata, the phyla we belong to, has its roots in the Cambrian, but you probably wouldn’t see much that you recognised around that time. Obviously at that time all life was still in the water, so there were nothing in the reptiles, amphibians, mammals or birds. Even the fish were not really much like the ones we see – they had a lot of development to do before they would be the flashy silver things we see today. Sadly no frickin’ sharks with frickin’ laser beams attached to their frickin’ heads seem to have evolved either.
Also, as more examples are found, and as analysis techniques get better, expect there to be a certain amount of settling down of ideas and phylogenies. If there is one thing that taxonomist (those who attempt to pigeon-hole life forms) agree upon, its that they generally disagree about exactly what pigeon-hole is appropriate.
So what did life do next?