Friday, 4 July 2014

On The Origin Of The Eukaryotes

The Francevillian Group fossils: early eukaryotes, algal mats or pyrite pseudomorphs?
The origin of the eukaryotes is one of the most hotly debated subjects in palaeontological history. Complex cells revolutionised the biological world, and pinpointing exactly when they first evolved, has been an aim of many evolutionary biologists and palaeontologists. The oldest putative evidence comes from the Francevillian Group in Gabon, in the form of the fossilised impressions.

1.1 billion year old undisputed eukaryote microfossils from Loch Torridon
Yet their identity as such is disputed. There are theories that the Francevillian Group fossils are nothing more than the remains of algal mats which were draped over stones, resulting in complex shapes and markings which have been misinterpreted as the imprints of multicellular eukaryotes.

What is more, studies of mitochondrial, chloroplast and nuclear genes of various eukaryote groups suggest that complex cells appeared no more than 1.2 billion years ago. Molecular evidence invariably sets the parameters for fossil evidence and so it seems even less likely that the Francevillian Group specimens are eukaryotic in nature. There is also the matter of how they fit into the rest of the fossil record.

The next distinct appearance of eukaryotes is a billion years later, leaving a massive, inexplicable gap in the record. The fossils themselves are also far simpler, namely microscopic clusters of cells which display very little organisation. This is in contrast to the organised, macroscopic Francevillian Group fossils. The trend in complexity is the wrong way round. It is another few hundred million years before we see organisms whose complexity matches that of the Francevillian Group specimens

On the other hand there are undisputed eukaryote fossils from 1.1 billion year old rocks at Loch Torridon which are multicellular, but at a microscopic level. These display the level of complexity one would expect if complex cells had originated 100 million years earlier, as well as existing at a time when multicellularity would have been an advantage in the toxic, oxygen and nutrient deprived waters of Earth's Precambrian oceans. In this scenario the fossil evidence neatly slots together and complements the environmental conditions of the time - without violating the molecular constraints provided by genetic evidence. The Francevillian Group fossils cause more problems than they solve and so it seemed highly unlikely just a few months ago that they could be fossils of early eukaryotes.

The fossils and CT reconstructions of members
of the 2.1 billion year old Gabonionta
Yet a new study just published upholds the interpretation of the Francevillian Group fossils as eukaryotic. Dolf Seilacher, from Yale University, famously interpreted the impressions as inorganic pseudofossils made from unusual growths of pyrite.

While it is true that many of the fossils were composed of pyrite, researchers from several French institutions used ion probe analysis to determine if the impressions were organic in origin and not simply mineral growth.

Since their discovery in 2010, the team have been cataloguing the impressions, categorizing them into several morphotypes: lobed, circular and lobed. The sheer range and diversity of forms makes them difficult to dismiss as algal mats draped over stones.

The researchers suggest that the Francevillian Group fossils constitute an entire ecosystem of early eukaryotes, now called the Gabonionta, which inhabited a shallow, well oxygenated river delta. Indeed many of the forms identified bear a resemblance to other complex organisms seen later in the fossil record, such as Nemiana or Horodyskia.

As the team were also able to identify microscopic members of the Gabonionta, it is possible that the 1.1 billion year old Loch Torridon fossils do not represent a break in complexity, but are simply the microscopic representatives of a eukaryote fauna whose macroscopic members were not preserved. The Precambrian fossil record is notoriously incomplete.

This raises the issue, however, as to why complex cells maintained simple body plans for over a billion years until the Cambrian Explosion. In the scenario where eukaryotes appeared 1.2 billion years ago, the fossil record provides a steady increase in complexity; while environmental pressures may help explain the slow rate of change. The stagnation of the oceans during the Boring Billion and two ice ages would certainly have put the brakes on eukaryote evolution. What is more the increase in complexity represented by the Gabonionta (assuming they are eukaryotes) and the increase represented by the Vendobionta (the Ediacara biota) both coincide with increases in the level of atmospheric oxygen, providing a mechanism for the development of new, complex body plans.