On the Origins of Multi-cellular Life
Did it start in South Australia?
Notes by Craig Robertson
My interest in palaeontology goes well back to my youth. However I have found my interest in the history of life on Earth has intensified with age. Like many of course I am also interested in the possibility of life elsewhere in the universe. The hunt for it has also intensified in recent years. My bet is life will be found elsewhere - Mars, maybe the moons of the gas giants, or even interstellar planets. But I also think it will most likely be unicellular only; microbes - algae, bacteria, fungus - or virus grade organisms. If that comes about the real mystery about life will come to the fore: How did multi-cellular life get started so all the plants and animals could evolve?
This interest has been stimulated in recent years by trips to the Flinders Ranges in South Australia. The oldest fossils of multi-cellular organisms were found in the Ediacara area by geologist Reg Sprigg in the 1940s. It took some time for his finds to be recognized, but similar fossils have since been found elsewhere, for example, in Canada and Siberia. Sprigg and other geologists achieved something for Australia by having the rock strata recognised by the International Union of Geological Sciences as the Ediacaran, a globally valid subdivision of geological time forming the upper layer of the Neoproterozoic.
The "Golden Spike" in a creek bed in the Flinders Ranges marks the base of the Ediacaran Period, a globally recognized sub-division of the Neoproterozoic, itself a sub-division of the long Proterozoic Eon.
|(Left) Global Stratotype Section and Point (GSSP), aka the "Golden Spike", marking the base of the Ediacaran in the Flinders Ranges.
Keeping it simple, the geological history of the Earth is divided into just three eons according to the history of life: Archaean, Proterozoic and Phanerozoic. The first two see the emergence of single-cell organisms and their proliferation in various key groups over a timespan of a good three billion years. The Phanerozoic is when the fossil record really gets going and multi-cellular (metazoan) life surges in the 'Cambrian explosion', the first period of the Palaeozoic era. The fossil record is pretty good from here on, mainly because multi-cellular creatures living in the sea formed shells or skeletal structures.
Nothing much was known about the pre-Cambrian until Reg Sprigg made his discoveries. However it is obvious multi-cellular organisms must have already been reasonably well developed before they could secrete shell. The questions are around just when and how. The where is at least in the sea, but is open about whether organisms were benthic or pelagic, or possibly both during their life cycles.
So far there have been found a variety of fossils - just impressions made in fine sandy sediments - that are clearly from multi-cellular organisms, although there is still often debate about their nature. For example there are round disc-like organisms that may have been flat blobs on the sea floor, or actually hold-fasts for creatures that grew up into the water column, filtering it for food, their bodies lost in the fossilisation process.
My question was along the lines of what would make a group, a large group, of single-cells come together, stick together and then start differentiating their functions in the whole organism. There are of course colonial types, but they were around for hundreds of millions of years too. So why would one group start sucking food in and sharing it around and another group spew out the waste, ie form a primitive gut. Sponges are like this; they are the basal group among living metazoans.
Ediacaran fossils on display at Parachilna
|In the 1980s and 1990s geologists working in South Australia discovered the Acraman meteorite impact crater and its effects on the sediments forming in a shallow sea where the Flinders Ranges now stand, ie close to the Ediacaran fossil finds, and dated at about 580 million years, during the Ediacaran period in the late pre-Cambrian. Some interesting speculation arose as to whether it was possible the impact itself might have shaken up the single cells and started something multi-cellular.
|(Left) Fossilized acritarchs, colonial organisms, in Brachina Gorge, Flinders Ranges, South Australia
The question is still unresolved. Further research seems to make it unlikely. For one thing there is some evidence of multi-cellular organisms pre-dating the Acraman impact, but the fossil record is weak. There were abundant colonial microbes for hundreds of millions of years beforehand, and they may have had different phases of their life-cycle crowded together in a tight space. But they would need to be communicating with one another in ways that would lead to a primitive nervous system. Also, while having single cells bind together on some sort of lattice is important, ie the lattice or other 'glue' is important, it may not be a matter of binding otherwise fully independent cells. It may have happened from the other direction so to speak.
The history of fairly complex single-cell colonial organisms may go all the way back to the Archaean. But during the long gestation of single-cell organisms - bacteria (prokaryotes) - the evidence is that they evolved into more and more complex cells with nuclei (eukaryotes). This was driven at least in part by predation; it is likely that cell nuclei were the result of smaller cells invading larger ones and eventually establishing symbiotic relationships. There is a vast array of cell structures and complexity in the microfossil record. They were active creatures, engaging with their environments. There are fossils of relatively complex sponge-grade organisms before and after the impact.
It is quite conceivable that eventually when some variety of cell divided, instead of the cells separating, they stayed bound together, perhaps in response to some environmental change in the chemistry of the sea, then began to differentiate.
The first structure may have been a simple cup or open hollow of some sort, the beginnings of a gut. There may well have been collaborating microbes inside it. As when the first microbes formed, there was a boundary with channels across it. It is also possible that as early as the second generation division, when two cells split into four, that one cell could hive off as a proto-laval stage. From this simple beginning numerous multi-cellular structures could emerge and multiply. Animals all share some attributes so it is probable this may only need to have happened once, although there were probably multiple transitions. There is also a survival advantage in becoming too big to eat.
It is something that could happen any time in a way. The chemistry of the marine environment is the more plausible catalyst, however it may in turn be the result of global glaciation processes, the rapid erosion of a massive trans-Gondwanan mountain range, changes in salinity, single-cell activity - especially producing oxygen, probably by photosynthesising algae - and/or a violent Earth-shaking event. It is interesting that it took so long for multi-cellular life to get going compared to single-cell life, which seemed to appear as soon as the Earth was cool enough.
Victor Gostin, David McKirdy and George Williams, 2011. Ice, and Asteroid Impact & the Rise of Complex Life. Australasian Science, May, 2011.
There are many other papers relating to this research, in numerous journals it is becoming harder and harder for a layman to access. My motivation has flagged.
My thanks to Peter Crettenden, Swagabout Tours, Adelaide, for finding the Golden Spike and more Ediacaran fossils.
Updated: 21 March, 2022.
Text © Copyright Craig Robertson, 2021-2, except where otherwise attributed.
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