A few weeks ago a paper titled Life Before Earth was posted on the arXiv preprint repository. It came to my attention by way of this MIT Technology Review article and this blog post. The paper, using a rather simple extrapolation, argues that the apparent rate at which the complexity of terrestrial life increases suggests that its birth occurred approximately 9.7 billion years ago. Earth, in contrast, is around 4.5 billion years old. If their extrapolation is to be believed, then this discrepancy can only be resolved if terrestrial life is in fact of extraterrestrial origin. I will briefly summarize their argument, but I will not attempt to justify its validity. The original paper can be read here and is fairly accessible. The paper’s conclusions are consistent with a fact that has always puzzled me; the surprising complexity and maturity of what is known as the Last Universal Common Ancestor. It is this topic that I wish to focus on in this post.