How did life originate? Scientists have been studying the question for decades, and they’ve developed ingenious methods to try to find out. They’ve even enlisted biology’s most powerful theory, Darwinian evolution, in the search. But they still don’t have a complete answer. What they have hit is the world’s most theoretically fertile dead end.
When scientists look for life’s origins, they usually work in one of two directions. They work backwards in time through the record of organisms that have lived on Earth, or they work forward from one of the many hypothetical prebiotic worlds in which life could have emerged.
When they work backwards, they travel through the fossil record, and through the branches of genetic relationships between species. They also look for geochemical signatures that mark life’s presence in the distant past. Somewhere at the end of the line lies life’s oldest ancestor. This ancestor has acquired a name: LUCA, the last universal common ancestor. It also has a hypothetical nature and place in the biological order of things: LUCA is a microorganism or group of microorganisms from which all life on Earth descends. Though scientists, such as the molecular biologist William Martin of Heinrich Heine University in Düsseldorf, and his team, have been able to infer some part of LUCA’s genetic profile, they don’t have a complete portrait. They also can’t see beyond LUCA: LUCA isn’t necessarily the first life, and scientists can’t see what other life forms could have cropped up before it. Ultimately, LUCA is the living system that scientists identify to say that, at least once, somewhere, spontaneously, life got its start on Earth.
To emphasise that any life before LUCA is currently unknowable, scientists call LUCA a phylogenetic event horizon. Phylogeny is the study of genetic relationships between species over evolutionary time; it allows scientists to trace the history of life. The term ‘event horizon’, in contrast, hails from astrophysics, and refers to the threshold around a black hole. Beyond this threshold, escape velocity surpasses the speed of light. Since nothing can travel through space faster than the speed of light, there’s no way to witness any event that takes place there. So too with LUCA: it marks a biological boundary beyond which no observer can see. Since there’s no record for phylogenetic analysis to work upon before LUCA, scientists can’t follow the biological record there.
Where to go from here?
It’s in the face of phylogenetic limits that scientists are peering into the deep past, nonetheless, and theorising hypothetical beginnings anew. [Continue reading…]