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Researchers at the University of Bristol have proposed a date for the origins of chloroplasts — the cellular engines that make energy from the sun via photosynthesis — that, if correct, could confirm or disprove several theories about the earliest life on Earth.

Why it matters: Scientists debate the origins of chloroplasts and, relatedly, what gave rise to complex life on Earth. This research validates some controversial ancient fossil finds, and gives us a better idea of where and how life arose — and what to look for in the search for it elsewhere in the universe.

Scientists know that the engines that help plants make energy from the sun via photosynthesis used to be their own organisms, until they were swallowed by another type of bacterium and formed a symbiotic relationship with them. Most agreed that organism was some type of cyanobacteria, but the species and the date remained open questions.

What they found: The researchers found the chloroplast lineage split from the rest of the cyanobacteria 2.1 billion years ago. They evolved on their own for roughly 200 million years before symbiosis occurred. According to their analysis, the chloroplast's closest relatives are Gloeomargarita bacteria.

What was known: The oldest commonly accepted eukaryote fossil is Bangiomorpha, a 1.1 billion year old red algae. But fossils described earlier this year were controversially dated to 1.6 billion years old, and appear to have chloroplasts and characteristics of eukaryotes. Those fossils, if they were interpreted correctly, pushed back the potential origin of eukaryotes by several hundred million years. Debashish Bhattacharya, an evolutionary biologist at Rutgers University who is familiar with both this study and the fossils, tells Axios that this research "essentially confirms" both Bangiomorpha and this years' ancient fossil find.

What they did: The researchers looked at the genomes of 49 different cyanobacteria and 70 eukaryotes (organisms that, like humans, have genes in a self-contained nucleus). They looked at 26 genes shared by both groups, and used a molecular clock, which estimates time based on mutations in genes, to create and date how the species are related and when they diverged.

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