The study challenges previous research, which on the basis of fossil evidence had theorised that the first placental mammal arose after the dinosaurs died out.

Placental mammals today include humans and all other mammals except those that lay eggs or have pouches (marsupials).

"When dinosaurs died out, many ecological niches became vacant, and placental mammals took over," study lead author Mario dos Reis told Discovery News.

“The placental ancestor diversified and evolved into the modern mammals we see today such as rodents, deer, whales, horses, bats, carnivores, monkeys and ultimately humans,” added dos

The lead author further said, “If dinosaurs had not died out, then placental mammals may not have had the opportunity to diversify the way they did, and our own species would not have evolved!”

Researchers analysed 36 complete mammal genomes together with information from the mammal fossil record. The results determined placental mammals originated in the Cretaceous period.

Mario dos Reis explained that the DNA of organisms accumulates changes called mutations at a constant rate in time. This is referred to as the ‘molecular clock’.

For example, certain DNA in humans and other apes mutates at a pace of about one percent every 10 million years.

Researchers estimated the number of mutations that accumulated in each mammal lineage, corrected for the flaky clock, and together with ages from known fossils estimated the age of the placental ancestor.

Previous research has shown that this animal was small, nocturnal and pretty scrappy. It either lived far away from the asteroid impact site that caused the extinction of non-avian dinosaurs, or was somehow saved because of its size, habitat and/or lifestyle.

About 70 percent of all species died during the mass extinction event 66 million years ago, with even some mammals, birds and plants going extinct then. The study is published in journal Biology Letters.


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