Genomics holds the key to unravelling history of life
A new study led by scientists from the universities of Bristol and Bath has used a combination of genomic and fossil data to explain the history of life on Earth, from its origin to the present day.
Palaeontologists have long sought to understand ancient life and the shared evolutionary history of life as a whole, but the fossil record of early life is extremely fragmented. Its quality significantly deteriorates further back in time towards the Archaean period, more than 2.5 billion years ago - the time when the Earth’s crust had cooled enough to allow the formation of continents, and the only life forms were microbes.
There are very few fossils from the Archaean period, and they cannot be confidently assigned to the evolutionary lineages with which scientists are familiar. Fossil evidence is so fragmented and difficult to evaluate that new discoveries and reinterpretations of known fossils have led to a rise in conflicting ideas about the timescale of the early history of life.
“The problem with the early fossil record of life is that it is so limited and difficult to interpret”, explained Holly Betts, lead author of the study, from the University of Bristol’s School of Earth Sciences, “careful reanalysis of the some of the very oldest fossils has shown them to be crystals, not fossils at all.”
However, a team of scientist from the universities of Bristol and Bath have adopted an alternative approaching to dating life.
“Fossils do not represent the only line of evidence to understand the past,” explained co-author Professor Philip Donoghue. “A second record of life exists, preserved in the genomes of all living creatures.”
By combining fossil and genomic information, the team are able to use an approach called the ‘molecular clock’. This approach is loosely based on the idea that the number of differences in the genomes of two living species (say a human and a bacterium) are proportional to the time since they shared a common ancestor.
By making use of this method the team were able to derive a timescale for the history of life on Earth that did not rely on the ever-changing age of the oldest accepted fossil evidence of life.
Co-author Professor Davide Pisani said: “Using this approach we were able to show that the Last Universal Common Ancestor of all cellular life forms (‘LUCA’) existed very early in Earth’s history, almost 4.5 Billion years ago - not long after Earth was impacted by the planet Theia, the event which sterilised Earth and led to the formation of the Moon.”
“This is significantly earlier than the currently accepted oldest fossil evidence would suggest,” Professor Pisani continued.
“Our results indicate that two ‘primary’ lineages of life emerged from LUCA (the Eubacteria and the Archaebacteria), approximately one billion years after LUCA.”
“This result is testament to the power of genomic information, as it makes it possible to discriminate between the oldest eubacterial and archaebacterial fossil remains.”
The study confirms modern views that the eukaryotes, the lineage to which human life belongs (together with the plants and the fungi, for example), is not a primary lineage of life.
Professor Pisani added: “It is rather humbling to think we belong to a lineage that is billions of years younger than life itself.”
This research was funded by the Biotechnology and Biological Sciences Research Council and the Natural Environment Research Council.
Notes to editors
Research paper: Nature Ecology & Evolution: Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin Betts H.C, Puttick M.N., Clark J.W., Williams T.A., Donoghue P.C.J., and Pisani D. DOI: 10.1038/s41559-018-0644-x.
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