The research led by the University of Bristol was published in the scientific journal Nature 3 December, the research indicates that complex organisms evolved long before there were substantial levels of oxygen in the atmosphere, something which had previously been considered a prerequisite to the evolution of complex life.
Prokaryotes as the only forms of life
'The earth is approximately 4.5 billion years old, with the first microbial life forms appearing over 4 billion years ago. These organisms consisted of two groups – bacteria and the distinct but related archaea, collectively known as prokaryotes,' said co-author Anja Spang , from the Department of Marine Microbiology & Biogeochemistry at the Royal Netherlands Institute for Sea Research (NIOZ). Spang had previously made important discoveries in this field and was involved in the analysis and interpretation of the results within this new study.
Prokaryotes were the only form of life on earth for hundreds of millions of years, until more complex eukaryotic cells including organisms such as algae, fungi, plants and animals evolved.
Previous ideas were speculation
Davide Pisani , Professor of Phylogenomics in the School of Biological Sciences at the University Bristol, explained: 'Previous ideas on how and when early prokaryotes transformed into complex eukaryotes has largely been in the realm of speculation. Estimates have spanned a billion years, as no intermediate forms exist and definitive fossil evidence has been lacking.'
Molecular clocks
However, the collaborative research team has developed a new way of probing these questions, by extending on the 'molecular clocks' method which is used to estimate how long ago two species shared a common ancestor.
'The approach was two-fold: by collecting sequence data from hundreds of species and combining this with known fossil evidence, we were able to create a time-resolved tree of life. We could then apply this framework to better resolve the timing of historical events within individual gene families,' added co-lead author Professor Tom Williams in the Department of Life Sciences at the University of Bath.
The researchers collected evidence from more than a hundred gene families in multiple biological systems. In doing so, they focused on the characteristics that distinguish eukaryotes from prokaryotes, such as the transport of compounds within the cell via vesicles. With that information the team were able to begin to piece together the developmental pathway for complex life.
The cell nucleus existed before the mitochondrion
Surprisingly the researchers found evidence that the transition began almost 2.9 billion years ago – almost a billion years earlier than by some other estimates – suggesting that the nucleus and other internal structures appear to have evolved significantly before mitochondria. 'The process of cumulative complexification took place over a much longer time period than previously thought,' said author Gergely Szöllősi, head of the Model-Based Evolutionary Genomics Unit at the Okinawa Institute of Science and Technology (OIST).
The new scenario: CALM
The data meant the scientists have been able to reject some scenarios put forward for eukaryogenesis (the evolution of complex life), and their data did not neatly fit with any existing theory. Consequently, the team has proposed a new evidence-based scenario for the emergence of complex life they have called 'CALM' - Complex Archaeon, Late Mitochondrion.
A number of disciplines
Lead author Dr Christopher Kay , Research Associate in the School of Biological Sciences at the University of Bristol, explained: 'What sets this study apart is looking into detail about what these gene families actually do - and which proteins interact with which - all in absolute time. It has required the combination of a number of disciplines to do this: palaeontology to inform the timeline, phylogenetics to create faithful and useful trees, and molecular biology to give these gene families a context. It was a big job.'
Mitochondria arrived later than previously thought
'One of our most significant findings was that the mitochondria arose significantly later than expected. The timing coincides with the first substantial rise in atmospheric oxygen,' said author Philip Donoghue , Professor of Palaeobiology in the School of Earth Sciences at the University of Bristol.
'This insight ties evolutionary biology directly to Earth's geochemical history. The archaeal ancestor of eukaryotes began evolving complex features roughly a billion years before oxygen became abundant, in oceans that were entirely anoxic.'
Paper
' Dated gene duplications elucidate the evolutionary assembly of eukaryotes ' by C. Kay, A. Sprang, G. Szöllősi, D. Pisani, and P. Donoghue in Nature
