Scientists have just published compelling evidence that your great (great great...) grandmother was a shrew. Or rather, a shrew-like creature. A team of researchers discovered a new fossil species that they've named Juramaia sinensis, or "the Jurassic mother from China" — a tiny, primitive mammal that dates all the way back to 160 million years ago.
The researchers believe Juramaia to be the oldest known specimen of the so-called "eutherian" mammals — the distant ancestors of modern day placental mammals like us humans.
The team, which was led by Carnegie Museum of Natural History paleontologist Zhe-Xi Luo, discovered the remarkably well-preserved fossil discovered in northeast China.
Their finding is one of unusual significance. For years, DNA evidence has suggested that the evolutionary split of placental mammals from metatherian mammals (metatherian mammals being the ancient predecessors of modern day marsupials like kangaroos) occurred roughly 160 million years ago.
Until now, however, fossil evidence of this evolutionary divergence did not corroborate with DNA findings. Speaking to the discrepancy between genetic and fossil evidence in their recent paper — published in tomorrow's Nature — the authors explain the importance of their discovery:
Juramaia, at an age of 160 Myr, establishes a much older geological time for the split of the metatherian–marsupial and the eutherian–placental lineages than previously shown by the fossil record. The previously earliest eutherian record is Eomaia and the metatherian record is Sinodelphys, both about 125 Myr...Juramaia extends the first appearance of eutherians from these previous records by about 35 Myr...Therefore this new fossil serves to re-set the minimal age at 160 Myr for the basal-most diversification of marsupials and placentals, the two clades that collectively make up 99.9% of all living mammals.
In other words, prior to the discovery of Juramaia, the fossil record had only pointed as far back as 125 million years. The researchers' discovery therefore brings the fossil record almost directly in line with current DNA evidence.
"Understanding the beginning point of [placental mammals] is a crucial issue in the study of all mammalian evolution," said Luo.