New research from the Universities of Bristol and Bath suggests life originated on our planet a lot earlier than previously thought.
An artistic conception of the early Earth. Image credit: Simone Marchi / NASA.
Scientists have long sought to understand ancient life and the shared evolutionary history of life as a whole.
However, the fossil record of early life is extremely fragmented, and its quality significantly deteriorates further back in time towards the Archean eon, more than 2.5 billion years ago, when the Earth’s crust had cooled enough to allow the formation of continents and the only life forms were microbes.
Now University of Bristol researcher Holly Betts and colleagues have used a combination of genomic and fossil data to explain the history of life on Earth, from its origin to the present day.
“There are few fossils from the Archean and they generally cannot be unambiguously assigned to the lineages we are familiar with, like the blue-green algae or the salt-loving Archaebacteria that colors salt-marshes pink all around the world,” Betts said.
“The problem with the early fossil record of life is that it is so limited and difficult to interpret — careful reanalysis of the some of the very oldest fossils has shown them to be crystals, not fossils at all.”
Fossil evidence for the early history of life is so fragmented and difficult to evaluate that new discoveries and reinterpretations of known fossils have led to a proliferation of conflicting ideas about the timescale of the early history of life.
“Fossils do not represent the only line of evidence to understand the past. A second record of life exists, preserved in the genomes of all living creatures,” said co-author Professor Philip Donoghue, from the School of Earth Sciences at the University of Bristol.
“Combining fossil and genomic information, we can use an approach called the ‘molecular clock’ which 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,” added co-author Dr. Tom Williams, also from the School of Earth Sciences at the University of Bristol.
By making use of this method the researchers 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.
A timescale for the evolution of life on Earth. Tip labels are shown for Eukaryota (gray), Archaebacteria (red) and Eubacteria (blue). The purple bars denote the credible intervals for each node. Red dots highlight calibrated nodes, and corresponding black dots highlight the age of the minimum bound of its corresponding calibration. The phylogenetic relationships of the mitochondrion within Alphaproteobacteria are still debated, and it is unclear whether the free-living ancestor of the mitochondrion was a crown or stem representative of this group. The red bar above the crown eukaryote node denotes the time period during which the mitochondrial endosymbiosis may have occurred. The green bar denotes the time during which the plastid endosymbiosis may have occurred. Important events in Earth and life history are indicated along the base of the figure. Mesoprot – Mezoproterozoic; Neoprot – Neoproterozoic. Image credit: Betts et al, doi: 10.1038/s41559-018-0644-x.
“Using this approach we were able to show that the Last Universal Common Ancestor of cellular life (LUCA) existed very early in Earth’s history, almost 4.5 billion years ago — not long after Earth was impacted by the protoplanet Theia, the event which sterilized Earth and led to the formation of the Moon,” senior author Professor Davide Pisani, from the Schools of Earth and Biological Sciences from the University of Bristol.
“This is significantly earlier than the currently accepted oldest fossil evidence would suggest.”
“Our results indicate that two ‘primary’ lineages of life emerged from LUCA (Eubacteria and Archaebacteria), approximately 1 billion years after LUCA.”
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.
“It is rather humbling to think we belong to a lineage that is billions of years younger than life itself,” Professor Pisani said.
The findings are published in the journal Nature Ecology & Evolution.
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Holly C. Betts et al. Integrated genomic and fossil evidence illuminates life’s early evolution and eukaryote origin. Nature Ecology & Evolution, published online August 20, 2018; doi: 10.1038/s41559-018-0644-x
