When we think of the Ice Age we usually think of the Pleistocene era, which ended roughly 12,000 years ago. But that was a balmy summer compared to what happened about 700 million years ago, when basically the entire planet iced over in a period often called "Snowball Earth." Now researchers have found fossil fats that help fill in the story of how life bounced back after this global cataclysm, and how we humans might owe our very existence to them.

The research team started by analyzing molecules in rocks in Brazil dating back 635 million years, which is just after the Snowball Earth phase. Among them they found a previously-unknown biomarker, purified it and then used nuclear magnetic resonance to identify its chemical structure. This molecule, now named 25,28-bisnorgammacerane (BNG), was identified as a type of fat.

"All higher animal life forms, including us humans, produce cholesterol," says Lennart van Maldegem, first author of the study. "Algae and bacteria produce their own characteristic fat molecules. Such fat molecules can survive in rocks for millions of years, as the oldest (chemical) remnants of organisms, and tell us now what type of life thrived in the former oceans long ago."

But the team still wasn't sure what type of organism the BNG molecule had come from. To find out, they set out to see if they could find traces of it in other places. After studying hundreds of ancient rock samples, it turned out to be surprisingly common, with rocks taken from the Grand Canyon proving especially rich.

With more samples, the researchers were able to analyze other molecules and get a clearer understanding of the new molecule. After looking at other molecules that may have been precursors to BNG, the distribution of steroids, and stable carbon isotopic patterns, the team arrived at their prime suspect – heterotrophic plankton, some of the earliest predators of complex life.

"Unlike for example green algae that engage in photosynthesis and thus belong to autotrophic organisms, these heterotrophic microorganisms were true predators that gained energy by hunting and devouring other algae and bacteria," says van Maldegem.

The team says finding such ancient evidence of predation in plankton shows an important step in the evolution of the food chain. Importantly, these plankton would have thinned out the numbers of bacteria, which were the dominant lifeform of the time, allowing more complex organisms like algae to emerge. From those, larger lifeforms could arise, including the lineages that eventually led to humans and all other animals.

"Parallel to the occurrence of the enigmatic BNG molecule we observe the transition from a world whose oceans contained virtually only bacteria, to a more modern Earth system containing many more algae," says van Maldegem. "We think that massive predation helped to 'clear' out the bacteria-dominated oceans and make space for algae."

The findings fit in nicely with the story told by the current Snowball Earth hypothesis. So far, it's believed that the beginning of plate tectonics over 700 million years ago had understandably catastrophic consequences on the planet, as the cracking crust led to an increase in seismic and volcanic activity. That in turn created a global glaciation event.

After 50 million years or so, it's thought that the melting snow and ice washed huge amounts of nutrients into the oceans. That in turn would have given the early lifeforms like planktonic algae a decent leg up – some scientists call this period the "rise of the algae." Over the next 15 million years or so, some of this plankton turned predatory, paving the way for a complex food web that eventually gave rise to us.

The research was published in the journal Nature Communications.