A genetic archaeology project finds that two of life's deepest lineages may have arisen independently — opening the possibility that our planet was seeded more than once.
Every biology textbook tells the same origin story: life on Earth began once, in a hot, chemically rich environment, and everything we see — from bacteria to blue whales — is the result of a single, slow branching process. A new analysis of the deepest branches of the tree of life is beginning to question that assumption.
Researchers have been building a "genetic archaeology" of the earliest divergences in life, using long-read sequencing on microbes found in environments that have barely changed since the Hadean and Archean eons — deep hydrothermal vents, ancient hot springs and high-sulfur sediments. What they found is a pair of foundational lineages whose molecular signatures are so distinct that standard models of shared ancestry struggle to explain them.
Life as we know it is organised into two domains at the deepest level — Bacteria and Archaea — joined by a shared core of biochemistry. Most models assume that a single common ancestor, often called LUCA (the Last Universal Common Ancestor), split into these two domains. The new evidence does not discard LUCA; it suggests that LUCA may itself have been a product of two pre-LUCA lineages that met, exchanged chemistry, and fused into the ancestor of everything that follows.
The signatures come from a handful of genes that every living cell depends on — ribosomal machinery, energy-transfer proteins, and the enzymes that copy genetic information. In some cases, the bacterial and archaeal versions of these genes are not just different; they are arranged in ways that only make sense if they evolved separately before being stitched together.
If life started twice, the implication is profound. It would mean that the chemical steps from non-living matter to a self-replicating cell are not as rare or as uniquely tuned as we assumed. On a planet with abundant water and energy, the leap to life may be almost a given. For the search for life elsewhere in the solar system — on Europa, Enceladus or the early Mars — that is a much warmer signal than the textbook version allows.
The researchers caution that the debate is still open. Alternative explanations for the molecular patterns exist, and the fossil record of early life is too thin to settle the question on its own. But the genetic evidence is now strong enough that "two origins" has moved from speculation into a serious working hypothesis.