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According to a paper released Monday in Nature Ecology & Evolution, the still-unidentified microbe that scientists believe is the ancestor to all cellular life on Earth was born sometime before 3.9 billion years ago. As it turns out, the last universal common ancestor — LUCA for short — emerged even earlier than scientists once believed.
Scientists previously pegged the LUCA's birth to a period 3.5 to 3.8 billion years ago, but the new evidence examined in the study0 suggests it happened one hundred million years earlier. The researchers from the Universities of Bristol and Bath determined the LUCA's new age using the concept of the "molecular clock," which does away with all of the issues with relying on fossils to build Earth's early-life timeline. With early life fossils, there are always older ones waiting to be exposed, which may seem exciting but makes creating an early-life timeline very difficult. The molecular clock, in contrast, uses differences in the genomes of individual species to tell how much time has passed since they shared a common ancestor. The basic idea is that the more mutations two species share, the more time has passed since their evolutionary paths diverged.
The team applied a variant of this approach to some of the oldest existing fossils ever found, hoping they'd reveal when LUCA was born. "We used a relaxed clock framework, which means that the branches across the [evolutionary] tree can have differing rates of evolution," explains first author and University of Bristol Ph.D. candidate Holly Betts to Inverse. Because the differences in age that the molecular clock technique gives are relative, she explains, "you then use fossil calibrations to anchor the tree in real time."
Source: https://www.inverse.com/article/48247-early-life-domain-molecular-clock
0DOI: 10.1038/s41559-018-0644-x
(Score: 3, Informative) by takyon on Wednesday August 22 2018, @09:00AM
If you read the article, the numbers aren't certain, and there would have been a gradual lessening of the impact rate. There's also a graph there that places the earliest life at 4 billion years ago.
If the interval between the chaos of bombardment and the first life forms is (relatively) very short, that's probably great news for life in the galaxy. It could just be stupidly common wherever surface water and other conditions are met.
Enceladus and Europa are a bit sketchier, probably with less ideal conditions for life, and they also wouldn't help us find life in other star systems, since there's no way a telescope would be able to confirm subsurface life on an exoplanet or exomoon. The search for extraterrestrial life will require us to look at the composition of atmospheres or directly image exoplanets to look for obvious signs of life, such as vegetation. That being said, confirming life on Enceladus or Europa would lower the barrier to entry, so to speak, and hint at life on (within) many trillions of worlds in the galaxy. Subsurface oceans appear to be very common in our solar system, and we have a chance to find life from Ceres to Pluto and beyond.
Once we start detecting life or a lack of life on multiple exoplanets, we can extrapolate from the sample and guess how many exoplanets in the galaxy have life. That will help fill out a lot of the Drake equation. And early on, we could look for technological signatures such as atmospheric anomalies that could likely only be caused by industrial activity.
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