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MultiCellular Life Arose More than Once and is Far Older Than We Think

July 27, 2019 2 comments

During some background work for my series about why belief in anthropogenic climate change is a form of secular apocalypticism, I came across some interesting new evidence for multicellular life evolving far earlier than is common “scientific consensus”. But before we go there, let me first give you a bit of relevant background and explain certain concepts. First, life on earth is over 3.5 billion years old since the oldest undisputed microfossils of single-celled organism which resemble modern-day bacteria (Prokaryotes or Archaea) are at least that old. However, there is evidence for bacteria-like microfossils in even older rocks. And evidence for 4 billion year old life exists at more than one location. And yes.. there are still many in the “scientific community” who do not want to believe that life could have existed in the Hadean eon (4.6-4 billion years ago).

It is, however, important to understand that life on earth for the first 2 billion or so years was almost certainly unicellular. But how can we be so sure that this was the case. The simple answer to that question is, atmospheric oxygen, or more precisely its lack. See.. all existent life on earth has a common ancestor (DNA/RNA based), but that common ancestor was unlike almost all of the life on earth today, for a simple reason. Oxygen constitutes about 21% of our atmosphere now, but it was a trace gas (likely less than 0.1%) when the Earth formed 4.6 billion years ago and remained so until about 2.5-2.4 billion years ago. And we know that was the case, because there is a huge amount of evidence for the point in geological history when oxygen levels finally rose above 1%. And this change had a profound effect on the Earth, because it is linked to the longest period of global glaciation in Earths’s history– one lasting almost 300 million years.

But back to the type of microorganisms which lived on earth prior to the Great Oxygenation Event. The first ones were almost certainly Chemoautotrophs who survived under anaerobic conditions. They were followed by Haloarchea which use Rhodopsin rather than Chlorophyll for photosynthesis. Bacteria which use chlorophyll came later, but were almost certainly around by 3.5-3 billion years, based on the presence of fossil stromatolites and microfossils. The key connection between these distinct groups of bacteria is their ability to survive and grow without atmospheric oxygen. Moreover, cyanobacteria release oxygen during photosynthesis. So what happened to all the oxygen released by them for the first billion or so years of their existence? Well.. it reacted with available sinks of gaseous oxygen in the atmosphere, on land and in oceans. In other words, all that oxygen reacted with atmospheric methane, terrestrial deposits of iron and other readily oxidizable elements. Atmospheric levels rose only after these sinks were saturated.

While I am not going into the many effects of rising atmospheric oxygen on levels of methane and CO2, the short version is that levels of methane fell a lot, while those of CO2 increased. FYI- methane is a much more potent greenhouse gas than CO2 and the sun was less brighter in the past. All of this led to a long series of ice ages that lasted about 300 million years. But why do the levels of atmospheric oxygen matter? The simple reason is the energetic of anaerobic vs aerobic respiration is such that only the former can support multi-cellular life. This is not to say that rise in oxygen levels had no major effect on microbial life. Indeed, the rise must have killed most of the previously existing species of anaerobic microorganisms on earth. Today strict anaerobes exist only in certain environments such as under the soil, below ocean sediment, in decaying organic matter, bowels of ruminants etc.

Now let us, once gain, get back to the topic of this post- namely, ancient multicellular life. While bacteria can form mats and films with some external characteristics of simple multicellular organisms, they lack the defining feature- cellular differentiation. Multicellular organisms, you see, are defined by being Eukaryotic and exhibiting cellular differentiation. This is important to understand, because unicellular Eurkayotic organisms (with endosymbiotic mitochondria) have been likely around for about 2 billion years. And you will soon see why that approximate age for Eukaryotes becoming capable of aerobic respiration, through endosymbiotic mitochondria, is very relevant. Now let us talk about the history of scientific belief on when multicellular life first came into existence. Until 1959, geologists and paleontologists were certain that multicellular life came into existence at the start of the Cambrian era. But one pesky problem remained..

How did so many different phyla (many of which are still around) suddenly appear in the fossil record, without any precursors? But for decades, most scientists chose to ignore that question. The funny thing is.. odd shapes which looked like multicellular life-forms had been found in pre-cambrian rocks as early as 1868. But “scientific consensus” being what it is, such discoveries were ignored or explained away as bubbles or concretions for almost a century. It took the discovery of Charnia fossils in 1956-1958 for the evidence of pre-Cambrian life to become strong that it could no longer be ignored. Since then, fossils of multicellular life from the Ediacaran era have been discovered all over the world. While almost nobody today doubts these fossils to be of multicellular life, they raise more questions than answers.

While a few, such as Dickinsonia, can be tentatively assigned as animals (as opposed to plants), most Ediacaran biota does not resemble existing organisms. Even the body plan of many exhibits peculiarities such as fractal branching and radial symmetry, which are basically non-existent in animal phyla today. Also, they seem to lack a circulatory or digestive system, but have a far more structured body than sponges or jellyfish. To make matters more interesting, we do not know what they evolved from- especially given that the Earth had just emerged from an almost 100 million years long glaciation at the start of Ediacaran period. Nor can we can say with certainty, if they evolved into something which survived into the Cambrian period, which started about 540 million years ago. But wait.. there are even older fossils of multicellular organisms.

For the past few decades (1960 onward) the “scientific consensus” slowly accepted the reality of multicellular organisms during the Ediacaran period. But they still maintained that not much happened before 800 million years. Of course, there are multiple sites with fossils in the ‘boring billion’ between 0.8 and 1.8 billion years ago, including red algae from 1.6 billion years ago. And we cannot forget all those trace fossils, found all over the world, of what appear to be tunnels and tracks made by worm-like animals from around 1.5 to 1 billion years ago. However recent discoveries have pushed that date for multicellular life even further back in time.

In 2010, a French-Moroccan professor at University of Poitiers came across what appeared to be fossils of multicellular organisms from 2.1 billion years old black shales of the Paleoproterozoic Francevillian formation in Gabon, Africa. You might member that this was the time when Earth emerged from Huronian glaciation episode. The oxygen levels in the atmosphere had also finally reached about 2%, barely a tenth of today, but enough for primitive multicellular organisms. His group kept returning to that site and finding even more evidence of such fossils at that site, now known as Francevillian biota. Here is a figure from one of his papers in 2014.

and here is another.. FYI, many of disc shaped fossils are a few cm across.

While these fossils are not much to look at, in addition to being controversial because they go against “scientific consensus”, their existence is compatible with what we already knew about atmospheric conditions at that time. As it turns out, 2.1 billion years ago was immediately after the Huronian ice ages, when atmospheric oxygen levels had finally reached somewhere between 1 and 2% and Eukaryotes had recently gained endosymbiotic mitochondria. It is therefore within the realms of possibility for simple differentiated multicelllar animals such as slime molds, proto-fungi, jellyfish-like animals and proto-wormlike creatures to have evolved from unicellular Eukaryotes in the 50-100 million years years after the Huronian glaciation ended.

To be very clear, nobody is suggesting that they possessed dedicated circulatory or digestive systems like those seen in even the most primitive multicellular animals around today. But their overall size and morphology, in addition to the environment under which they were deposited strongly suggest they were multicellular. While we do not know if their descendants evolved into creatures such as those seen in the much later Ediacaran biota, that is irrelevant to the fact that they represent the evolution of multicellularity. My point is that multicellular life began much earlier than believers in the “scientific consensus” are willing to accept. Never forget that it is scientific theories which must adjust to observed reality, not the other way around. Here is a recent paper containing evidence for motility in some members of the Francevillian biota.

What do you think? Comments?