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| Dr Rosalie Tostevin. Field work in Namibia. Source: UCL Credit: Fred Bowyer |
The great thing about science is the way it all meshes neatly together. Although there may currently be gaps in our understanding these gaps are getting smaller and the piece that fills the gap almost always neatly fits in with what we already know.
Of course, there may be occasions when the piece doesn't quite fit and so we need to look again at the surrounding science, but in the long run, these conflicts are always resolved to smooth out the wrinkles and fill in the cracks.
And another one of these little gaps looks as though it might have been filled in. Despite the ever and unrealistically optimistic hope of creationists that the very next gap will be found to be where their god lives, no such god was found by science yet again. The answer was entirely consistent with what we already know - which is inevitable, when you think about it. It is inevitable because science is about reality and reality is consistent.
One of the little gaps in the know history of the evolution of life on earth is why it took so long for multicellular life to evolve. After about 3 billion years of the 3.8 billion that life has been around, the most complex things were single-celled algae. And then, quite suddenly in comparison, we had the first signs of skeletal animals.
At this point it might be worth looking again at Richard Dawkins' attempt to explain the sheer depth of the time-scale we are discussing here:
Fling your arms wide in an expansive gesture to span all of evolution from its origins at your left fingertip to today at your right fingertip. All the way across your midline to well past your right shoulder, life consisted of nothing but bacteria. Multi-celled invertebrate life flowers somewhere around your right elbow. The dinosaurs originate in the middle of your right palm, and go extinct around your last finger joint. The whole story of Homo sapiens and our predecessor Homo erectus is contained in the thickness of one nail-clipping.
As for recorded history; as for the Sumerians, the Babylonians, the Jewish patriarchs, the dynasties of the Pharaohs, the legions of Rome, the Christian Fathers, the Laws of the Medes and Persians which never change; as for Troy and the Greeks, Helen and Achilles and Agamemnon dead; as for Napoleon and Hitler, The Beatles and Bill Clinton, they and everyone that knew them are blown away in the dust from one light stroke of a nail file."
Richard Dawkins, Unweaving The Rainbow
The question of why it took so long for complex animal life to appear on Earth has puzzled scientists for a long time. One argument has been that evolution simply doesn’t happen very quickly, but another popular hypothesis suggests that a rise in the level of oxygen in the oceans gave simple life-forms the fuel they needed to evolve skeletons, mobility and other typical features of modern animals.
Although there is geochemical evidence for a rise in oxygen in the oceans around the time of the appearance of more complex animals, it has been really difficult to prove a causal link. By teasing apart waters with high and low levels of oxygen, and demonstrating that early skeletal animals were restricted to well-oxygenated waters, we have provided strong evidence that the availability of oxygen was a key requirement for the development of these animals. However, these well-oxygenated environments may have been in short supply, limiting habitat space in the ocean for the earliest animals.
So why did multicellular life take so long? Maybe the question should be, why did it 'suddenly' arise? Now a team of researchers led by Dr Rosalie Tostevin of UCL (now of the Department of Earth Sciences at Oxford University), based on fieldwork carried out in Namibia, think they have discovered the answer - an increase in the oxygen levels in water associated with the evolution of cyanobacteria. The team have published their findings in open access Nature Communications.Although there is geochemical evidence for a rise in oxygen in the oceans around the time of the appearance of more complex animals, it has been really difficult to prove a causal link. By teasing apart waters with high and low levels of oxygen, and demonstrating that early skeletal animals were restricted to well-oxygenated waters, we have provided strong evidence that the availability of oxygen was a key requirement for the development of these animals. However, these well-oxygenated environments may have been in short supply, limiting habitat space in the ocean for the earliest animals.
Dr Rosalie Tostevin, lead author,
quoted in UCLA press release.
quoted in UCLA press release.
The evolution of cyanobacteria, with their ability to synthesise sugar out of water and carbon dioxide was one of the major advances in life on earth but the byproduct of this process, oxygen, was toxic to life until living organisms evolved to use it.
The team believe they have shown a correlation between the incidence of skeletal animals and the amount of oxygen in their aquatic environment. High oxygen water appears to have been the domain of these complex animals while low oxygen water remained the domain of single-celled organisms. It's not yet clear why there should be this correlation but the coincidence of a simultaneous rise in the number of skeletal animals and the geological evidence of a rise in the level of atmospheric oxygen and with it an increased oxygenation of bodies of water has long been recognised.
Abstract
The oceans at the start of the Neoproterozoic Era (1,000–541 million years ago, Ma) were dominantly anoxic, but may have become progressively oxygenated, coincident with the rise of animal life. However, the control that oxygen exerted on the development of early animal ecosystems remains unclear, as previous research has focussed on the identification of fully anoxic or oxic conditions, rather than intermediate redox levels. Here we report anomalous cerium enrichments preserved in carbonate rocks across bathymetric basin transects from nine localities of the Nama Group, Namibia (~550–541 Ma). In combination with Fe-based redox proxies, these data suggest that low-oxygen conditions occurred in a narrow zone between well-oxygenated surface waters and fully anoxic deep waters. Although abundant in well-oxygenated environments, early skeletal animals did not occupy oxygen impoverished regions of the shelf, demonstrating that oxygen availability (probably >10 μM) was a key requirement for the development of early animal-based ecosystems.
R. Tostevin, R. A. Wood, G. A. Shields, S. W. Poulton, R. Guilbaud, F. Bowyer, A. M. Penny, T. He, A. Curtis, K. H. Hoffmann, M. O. Clarkson.
Low-oxygen waters limited habitable space for early animals.
Nature Communications, 2016; 7: 12818 DOI: 10.1038/ncomms12818
Copyright © 2016 The authors. Reprinted under terms of Creative Commons Attribution 4.0 International (CC-BY 4.0) license
So, it's beginning to look like we are here because our remote ancestors some 550 million years ago firstly needed to protect themselves against this new toxic waste that was polluting their environment and then learned how to use it to their advantage. Of course with it and with multicellularity came the need for respiratory and circulatory systems to bring nutrients and to remove waste from those cells that were no longer in direct contact with the water the organisms lived in.
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