A cache of exquisitely preserved 25 tiny bones, found in a coal mine in Gujarat, appear to be the most primitive primate bones yet discovered, according to an analysis by an international team researchers.
The bones, belonging to ancient, rat-sized, tree-dwelling primates, represent a very early stage of primate evolution, the researchers said.
"These are the best preserved and most primitive bones we have from the first five million years of primate evolution, but there's not enough evidence currently for us to figure out when these primates reached India or where they came from," said one of the researchers Kenneth Rose, Professor Emeritus at Johns Hopkins University School of Medicine.
Researchers from Des Moines University in the US; H.N.B. Garhwal University in Srinagar, Uttarakhand; Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand; Panjab University, Chandigarh; and Royal Belgian Institute of Natural Sciences in Brussels, Belgium, also contributed to the study published online in the Journal of Human Evolution.
Their assessment of the bones bolsters the idea that primates native to what is now India played an important role in the very early evolution of primates, mammals that include humans, apes and monkeys.
"All other primate bones found so far around the world clearly belong to one or the other of the two primate groups, called clades: Strepsirrhini and Haplorhini," Rose said.
"But many of the Gujarat bones show features that do not clearly belong to one clade or the other," Rose said.
This suggests that the little primates represent a very early stage of primate evolution, according to Rose and lead author Rachel Dunn, Assistant professor at Des Moines University.
The newly discovered group of 25 tiny bones, all from somewhere below the neck of the animals, are considerably more primitive than the oldest known primate fossil, Teilhardina, which first appears in deposits at the beginning of the Eocene, almost 56 million years old.
They are also more primitive than a relatively complete skeleton of the primate Archicebus, found recently in China and dated to about 55 million years ago, the study said.
Their analysis, Rose said, suggests the Gujarat primates are close descendants of the common ancestor that gave rise to the adapoids and omomyids found on the northern continents.
The Gujarat primates were adapted for climbing the tall dipterocarp trees of ancient rainforests but were less specialised than present-day leaping lemurs or slow-climbing lorises, according to the researchers.
Their limbs and joints suggest more generalised climbing, as in present-day mouse lemurs and dwarf lemurs.
Because of such features, the researchers are not sure which clade some of the bones belonged to, suggesting that they represent the most primitive primate anatomy known.