The finding, from a team led by Daniel Lahr of the University of São Paulo in Brazil, challenges existing theory about life during the time. Known as the latePrecambrianperiod, it was thought to feature only a small number of unicellular lineages, including undifferentiated proto-amoebae and photosynthetic algae known as stromatolites.
The new study revealed eight new ancestral lineages of Thecamoebae, the largest group in the amoeba domain. This newly discovered diversity has implications for understanding how microorganisms evolved on Earth.
"We show that diversification apparently already existed in the Precambrian and that it probably occurred at the same time as ocean oxygenation,” says Lahr.
Studying the evolution of life millions of years ago is not an easy task, as most microbial lineages do not leave a fossil record. Thecamoebae, and in particular the order Arcellids, however, are known as testates because they have a hard outer shell and leave behind vase-shaped microfossils (VSMs).
VSMs do not have DNA traces, as other larger fossils sometimes do. To construct the phylogenetic tree without DNA, the team studied 19 living species, relatives of the ancient theocambians, to analyse gene expression in living organisms, using a technique calledsingle cell transcriptome sequencing.
"We sequenced whole transcriptomes of arcellinid amoebae using live samples," Lahr explains. "This yielded several thousand genes and some 100,000 amino acid sites, or 100,000 datapoints giving us the phylogenetic tree, which had never been seen before."
Thefindings, published in the journalCurrent Biology, completely dismantle the previous classification of theocambians, and provide further clues into how the evolution of microbial life affected the Earth at the time.
“By increasing the resolution of how life evolved in the very remote past, we can understand a little better how life affects the planet's climate and even its geology,” Lahr notes.
“That will help us understand the climate changes we're currently experiencing.”