A tree has something in common with the weeds and mushrooms growing around its roots, the squirrels scurrying up its trunk, the birds perched on its branches, and the photographer taking pictures of the scene. They all have genomes and cellular machinery neatly packed into membrane-bound compartments, an organizational system that places them in an immensely successful group of life forms called eukaryotes.
The early history of eukaryotes has long fascinated scientists who yearn to understand when modern life started and how it evolved. But tracing the earliest eukaryotes back through Earth’s history has been difficult. Limited fossil data shows that their first ancestor appeared at least 1.6 billion years ago. Yet other telltale proofs of their existence are missing. Eukaryotes should produce and leave behind certain distinctive molecules, but fossilized versions of those molecules don’t show up in the rock record until 800 million years ago. This unexplained 800-million-year gap in early eukaryotic history, a crucial period when the last common ancestor of all of today’s complex life first arose, has shrouded the story of early life in mystery.
“There’s this massive temporal gap between the fossil record of what we think are the earliest eukaryotes and the first … biomarker evidence of eukaryotes,” said Galen Halverson, a professor at McGill University in Montreal.
There are many possible explanations for that paradoxical gap. Maybe eukaryotes were too scarce during that time to leave behind molecular fossil evidence. Or perhaps they were abundant, but their molecular fossils did not survive the harsh conditions of geologic time.
A recent study published in Nature offers an alternative explanation: Scientists may have been searching for the wrong fossilized molecules this entire time. When the study authors looked for more primitive versions of the chemicals others had been searching for, they discovered them in abundance — revealing what they described as “a lost world” of eukaryotes that lived 800 million to at least 1.6 billion years ago. [Continue reading…]