Incomplete fossil records make it difficult to accurately trace the emergence of the species. The genome was fully sequenced in 2007. The earliest fossil evidence of the species dates to around 4. 0–4. 5 million years ago and no subspecies are known. Scientists estimate that C. porosus is an ancient species that could have diverged from 12 to 6 million years ago. Genetic research has unsurprisingly indicated that the saltwater crocodile is related relatively closely to other living species of Asian crocodile, although some ambiguity exists over what assemblage it could be considered part of based on variable genetic results. Other relatively broad-snouted species such as mugger (C. palustris) and Siamese crocodiles (C. siamensis) seem to be the most likely candidates to bear the closest relation among living species.