The Origins Of Vertebrates Part 1: Let There Be Life
Have you ever wondered where we came from? Have you ever wondered where we and our other friends with backbones, lungs, arms and legs all started? If the answer’s yes you might know a little bit about this yourself, at least enough to know that I can’t very accurately begin at the origin of life itself if we want to find out how our ancestors grew a backbone, but I’ll give it my best shot, and within a few weeks perhaps we’ll all feel we know a lot more about our ancestors and their journey from non-life to being able to create the internet.
The story of vertebrates, like the story of all life, begins with non-life, and while if you wish to make an apple pie from scratch you should probably invent the universe, we’re going to skip that bit and head to an early earth and the environments from which the first living cells could have emerged from.
So the earth is about four point five billion years old (or 4.5 byo for short) and conditions compatible for life were seen on earth (well not seen but you get my meaning) pretty soon after that speaking about time on more of a geological scale; for example, 3.8 billion years ago (bya), as evidenced by sedimentary rock, there must have been liquid water on the earth, and as we all know, water is integral to life as we know it, although with any luck there is some life not as we know it out there and perhaps it can teach us more about the limiting factors for life origins.
Despite the presence of water however, the earth was still a pretty hostile place back in the day. There were dramatic temperature fluctuations that would swing far enough to make Al Gore soil himself, there was a constant barrage of asteroid impacts, constantly reshaping the surface of the earth, and there was no ozone layer to prevent much of the harmful radiation from the sun and elsewhere in space from making existing a much easier affair.
The asteroid impacts in particular make arguably the most popular theories of the origin of life (besides the one taught from the pulpit) seem more unlikely than in did at first glance – the primordial soup hypothesis.
This theory gained a lot of traction because of experiments performed proving that early conditions on earth could actually result in the formation of amino acids, simulated with electrodes and a ‘soup’ of chemicals that were likely to be present at the time. From this primordial soup amino acids may have developed into self-replicating systems such as RNA molecules, which may have eventually become compartmentalised within membranes and been able to catalyse their own synthesis (be able to trigger the creation of replicates of themselves, a stage necessary for evolution to kick off).
Perhaps I’m getting a bit ahead of myself here, before I go I want to talk about hydrothermal vents, or black smokers.
From the picture above you can see why this kind of place is particularly interesting from a modern example of a hydrothermal vent. Not only would these places be free from much of the carnage taking place on the earth’s surface, being in the depth of the ocean and all, but there is life down there, in one of the places we would least expect it today. Settled in between water that’s too hot for life and water that’s too cold for life, far from the light of the sun, these places support life regardless. In fact the clay precipitates in these chambers may have even allowed for catalysis of molecules such as RNA.
In fact, the progression from self-replicating molecules to the first cells may have occurred entirely within these sorts of systems, getting their energy from chemosynthesis, probably using mainly hydrogen as a fuel and carbon dioxide as their source of, well, carbon, resulting eventually in a little organism called LUCA, or our Last Universal Common Ancestor; the organism from which all life on earth is descended, and all while the earth was still anoxic. Ah, for a world without oxygen.
At some point later the ancestors of today’s archaea and today’s bacteria split off from one another, and from one of these groups (I have heard archaea suggested more than bacteria) there came an event, an event called endosymbiosis where the mitochondria (a then bacteria) was engulfed by, or possibly acting as a parasite to the aforementioned single celled organism, married it into a symbiotic relationship, from which its descendants were born with mitochondria already inside them. These sub-cellular energy factories could allow the host cells to grow much bigger, and even potentially into multicellular life, and it is from this lineage that all eukaryotes (organisms with cellular nuclei) and thus the vertebrates, are descended.
Okay, next time we’ll journey through multiple-celled organisms up into the animals and even closer to that backbone everyone keeps telling me I need to grow. See you then!
[image credits: yecheadquarters, pandasthumb, brh]