Sunday, 28 July 2013

The Oldest Life On The Land

For billions of years, the oceans were the domain of life. Only here were conditions right for creatures to flourish. The land was completely barren: an endless desert of dry, crumbling mountains, with the cracked remains of ancient lava flows. While some isolated colonies of bacteria might have existed on the coastline of the first continents or at the edges of thermal pools and lakes, it was only recently that life made its move onto the land.

An artist's impression of the Rhynie Chert, the
oldest terrestrial ecosystem on the planet
In 410 million year old rocks near the village of Rhynie in Aberdeenshire, Scotland; palaeontologists uncovered fossils of the oldest terrestrial ecosystem on the planet, a jumbled mix of primitive plants, fungal strands and bizarre arthropods. From these we can be sure that life was well established on land 410 million years ago. Others showed that living organisms had made brief ventures into the dry, oxygen-filled atmosphere. Yet exactly how these pioneers fared, however, or when creatures began to creep onto the land was unknown.

Whatever the case, it was thought that the invasion of the land was a fairly recent event. Yet earlier this year, a team of palaeontologists led by geologist Gregory J. Retallack, from the University of Oregon, uncovered from 2.2 billion year old Precambrian cliffs, composed of compacted prehistoric soils known as palaeosols, at the Hekpoort Formation near Waterval Onder, South Africa, a series of small fossils far more complex than anything else from that time in the Earth's history.

Demonstrating that they were fossils was a technical triumph because they were too big to be seen in a standard microscopic slide. The samples were imaged using powerful X-rays from a cyclotron, a particle accelerator, at the Lawrence Berkeley National Laboratory in California. The images enabled a three-dimensional restoration of the fossils' form. Named Diskagma buttonii, which means 'disk shaped fragment of Andy Button,' they are about the size of a match head and similar in shape to an urn with a tubular, hollow body enclosed at one end and open at the other, with a small stem at the base.

A reconstruction of the 2.2 billion year old
 fossils of Diskagma buttonii with a scale bar
What is more, these tubular individuals were joined together by thread-like structures, creating a web which sat on the surface of the soil, based on their position within the rock layers. 'They certainly were not plants or animals Geosiphon, a fungus with a central cavity filled with symbiotic cyanobacteria.,
but something rather more simple,' said Retallack, professor of geological sciences and co-director of palaeontological collections at the University of Oregon's Museum of Natural and Cultural History.

'There is independent evidence for cyanobacteria, but not fungi, of the same geological age, and these new fossils set a new and earlier benchmark for the greening of the land,' he said. 'This (gives) added significance because fossil soils hosting the fossils have long been taken as evidence for a marked rise in the amount of oxygen in the atmosphere at about 2.4 billion to 2.2 billion years ago, widely called the Great Oxidation Event.'

It is likely that the rise in oxygen was the trigger which resulted in these creatures or possibly their ancestors making the move onto dry land. Exactly what the structures represent is mysterious. Retallack has proposed that they might represent relatives of Geosiphon based on their morphology. Even so, it is difficult to identify what they truly are. If Retallack is right, then they are eukaryotes, the proper name given to complex organisms. This, however, presents a problem in light of a recent genetic study conducted by researchers from the University of California, Berkeley.

Is it possible that these fossils represent the oldest eukaryotes on the planet?
Using the genes found in various eukaryotic organelles, they found that eukaryotes first evolved 1.2 billion years ago. This would suggest that Retallack's fossils are not of complex creatures as they are older than the genetic date. However, their complexity and similarity to modern day eukaryotes cannot be ignored. As a result, we have one of those rare instances where the genetic and fossil evidence are not concurrent.

One possibility is that eukaryotes evolved twice. This, however, is highly unlikely. The other is that the genetic evidence is wrong, again unlikely. Time and time again, genetics has revealed the truth. By contrast, the fossil record is imperfect. Yet I reiterate: these fossils are too complex to ignore. Simple cells, or prokaryotes to give them their proper name, cannot create such structures by themselves. Only eukaryotes, or a combination of prokaryotes and eukaryotes can do this. For now, we shall have to wait on new evidence before we can truly say what these fossils are and what they tell us about the origin of complex cells.