While the experiments carried out by Stanley Miller and others who have built upon his work show that life may have arisen from a primordial soup, that possibility remains theoretical. There is no evidence for pre-cellular life on Earth; what's more, critics of the RNA world hypothesis point out that the experiments that support the concepts were conducted with biologically created RNA. RNA can act as both a template for self-replication and an enzyme for carrying out that process, but these findings have been carried out in controlled laboratory experiments. This doesn't necessarily prove such delicate actions could happen in the seas of the ancient Earth.
For reasons like these, the RNA world hypothesis has been largely abandoned by proponents of abiogenesis in favor of other hypotheses, like the simultaneous development of both proteins and genetic templates or the development of life around undersea vents similar to those currently inhabited by today's extremophiles. But there is one criticism that any abiogenesis hypothesis has difficulty overcoming: time. DNA-based life is thought to have developed on Earth beginning around 3.8 billion years ago, giving pre-cellular life forms about 1 billion years to carry out random processes of encoding useful proteins and assembling them into the precursors of cellular life [source: Discovery News]. Critics of abiogenesis say that simply isn't enough time for inorganic matter to become the theorized precellular life. One estimate suggests it would take 10^450 (10 to the 450th power) years for one useful protein to be randomly created [source: Klyce].
This is one obstacle that makes panspermia an attractive explanation: It doesn't explain the origin of life, merely the origin of life on Earth. Panspermia hypotheses don't necessarily contradict abiogenesis; they merely shift the origin elsewhere. Yet the jury is still out on several important factors that must be in place for panspermia to be correct. Is it possible, for example, for microbial life to survive during the harsh conditions found in the trip through space, the entrance to Earth's atmosphere and the impact on Earth's surface?
Some recent hypotheses suggest that it needn't survive. One researcher postulates that dead scraps of DNA could have arrived on Earth via ballistic panspermia and were replicated through a kickstarted process similar to RNA world [source: Grossman]. Other researchers aim to scour Mars for fossil life and compare any genetic material to that found universally on Earth to determine relation [source: Chandler].
Yet if life on Earth did begin elsewhere and travel to our planet the question still remains: What is life's origin?