July 29, 2012 | 14
Thursday 26th July saw the launch of SciLogs.com, a new English language science blog network. SciLogs.com, the brand-new home for Nature Network bloggers, forms part of the SciLogs international collection of blogs which already exist in German, Spanish and Dutch. To celebrate this addition to the NPG science blogging family, some of the NPG blogs are publishing posts focusing on “Beginnings”.
Participating in this cross-network blogging festival is nature.com’s Soapbox Science blog, Scitable’s Student Voices blog and bloggers from SciLogs.com, SciLogs.de, Scitable and Scientific American’s Blog Network. Join us as we explore the diverse interpretations of beginnings – from scientific examples such as stem cells to first time experiences such as publishing your first paper. You can also follow and contribute to the conversations on social media by using the #BeginScights hashtag.
The first signs of life on earth appeared about 4.5 Ga (1 Ga is an American billion, ie. 109 years) ago. It’s not yet completely certain exactly how this life arose; hot volcanic mineral springs have been suggested, as have the more traditional lightning-struck primordial soups and (rather wonderfully) radioactive beaches. At any rate something happened which lead to a little membrane-bound ball with internal nucleic acids which, crucially, could replicate…
And then it was all over really, bar the evolution.
The atmosphere back then was very different, little oxygen and an abundance of carbon dioxide with plenty of methane being released into the atmosphere once the first life forms started eking out an entropy-defying existence. In order to get energy to power cellular processes you need to set up redox pathways, which involve cycles of electron donors and acceptors. The main electron donors around at the time were H2, H2S and CH4 and the main acceptor probably nitrogenous. Water, the electron donor used for photosynthesis, was around in abundance, but none of the little proto-life-blobs quite had the energy required to split it (or the physical proteins required back then either) so it mostly stayed unused.
Carbon dioxide levels went down, methane levels went up and the planet warmed up a little due to global warming. Things stayed like that for a billion years or so (1 Ga) and then something quite special happened, something that would have mindblowingly devastating affects on the life surrounding it.
Photosynthesis. The process by which carbon dioxide is fixed into usable sugars by the splitting of a water molecule. The process of photosynthesis produces oxygen, which is highly dangerous for cells; it can screw up the internal redox potential, create dangerous free-radicals and precipitate ions out into soluble forms. This means that from the point of view of every other organism the newly-evolved photosynthetic blobs were floating around spewing toxic gas into the atmosphere.
The arrival of this new resource (oxygen) lead to a change in the way organisms respired as well. Up until what is sometimes called the Great Oxidation Event most respiration was anoxic, probably similar to anaerobic respiration, or fermentation, in anaerobic bacteria around today. This process, while enough t0 keep life going, is around sixteen times less efficient than aerobic respiration. The proto-bacteria that managed to use the oxygen would therefore have gained a major energy boost.
This energy boost allowed the oxygen-using bacteria to go forth and multiply, leaving the anoxic bacteria clinging to the few environmental niches where no oxygen could penetrate. Some of these oxygen-using bacteria were swallowed up by larger cells who then used them as specialised intracellular breathing compartments. The bacteria became mitochondria, and the cells with mitochondria grew bigger and formed more intracellular compartments. They became eukaryotic cells, the kind of cells that all multicellular animals are made from.
So even now, when you breath, it’s ancient bacteria inside your cells that process the oxygen. The only part of the human cell that does oxidative-respiration is the mitochondria. Sure, the human part of the cell can produce small amounts of energy in the cytoplasm, but then the whole process is shuttled into the mitochondria in order to get the massive oxygen energy boost.
One biochemical trick that evolved around two billion years ago to take advantage of oxygen is still being used for respiration by all multicellular life on earth.
This post was largely taken from a previous one, over at my old blog.