Some scientists have hypothesized that Australian aboriginals received a portion of their DNA from an ancient hominid species called Homo erectus, which for a short time was contemporaneous with modern man. A recent study published in PNAS (Proceedings of the National Academy of the Sciences) set out to answer this question by analyzing mtDNA and Y-chromosome samples from aboriginals.
A total of 172 mtDNA and 522 Y-chromosome previously published and new sequences from aboriginal Australians and New Guineans were analyzed for mtDNA and Y-chromosome variation and were compared to the current world haplogroup tree. All of the mtDNA sequences were members of the M and N founder branches, and all of the Y-chromosome sequences fell into the C and F founder branches.
The results suggest that the Australian aboriginals are descendants of the same emigrant group that left Africa 50,000 to 70,000 years ago and populated Europe and Asia. At least from the small number of samples analyzed for this study, there does not seem to be any DNA contribution from Homo erectus.
The uniformity of the sequences suggests that once humans migrated into the region there was little other gene flow. This might explain why the Australian and New Guinean populations share phenotypic features that are unique to the region.
You can read more about this new study at National Geographic or NewScientist, or read the article online for free at PNAS. Additionally, Ron Scott at Scott Genealogy has provided a transcript (pdf) of an interview with Toomas Kivisild (one of the authors of the study and a name that many genetic genealogists will recognize).