Washington, Jan 15 ANI | 11 months ago

A new study suggests that an ancestor of humans -- albeit one that is at the root of our family tree -- shared the planet with dinosaurs.

According to the study, this ancestor, the first placental mammal, lived between 88.3 to 91.6 million years ago.

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

The study counters prior research, based solely on fossil evidence, which theorized this "mother of all placental mammals" arose after the dinosaurs died out.

The researchers instead believe that it preceded the non-avian dino die off and that we wouldn't even be here if the dinosaurs were still around.

Lead author Mario dos Reis told Discovery News that when dinosaurs died out, many ecological niches became vacant, and placental mammals took over.

He said that 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.

He added that 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.

He and colleagues Philip Donoghue and Ziheng Yang analyzed 36 complete mammal genomes together with information from the mammal fossil record. The results determined placental mammals originated in the Cretaceous.

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 1 percent every 10 million years.

The molecular clock is not perfect, however, and it runs a bit fast in some species and a little slow in others.

Dos Reis and his team therefore 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.

Based on earlier research, it's thought 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.

The first placental mammal might not have looked very human-like, but studies such as this do have important implications for us.

The study is published in the journal Biology Letters.

(Posted on 15-01-2014)

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