An ancestor of humans -- albeit one that is at the root of our family tree -- shared the planet with dinosaurs, a new study concludes.

This ancestor, the first placental mammal, lived between 88.3 to 91.6 million years ago, according to the study, published in the latest issue of Biology Letters. 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.

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“When dinosaurs died out, many ecological niches became vacant, and placental mammals took over," 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."

“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!" added dos Reis, a research associate in the Department of Genetics, Evolution and Environment at University College London.

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.

An artist's rendering shows the first placental mammal.Carl Buell

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," he said.

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.

About 70 percent of all species died out during the mass extinction event 66 million years ago, with even some mammals, birds and plants going extinct then.

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“To understand why the big lumbering behemoths went extinct and the gracile birds and mammals did not, we need to further explore the fossil record based on predictions shaped by our molecular analysis which, for instance, suggests the age intervals in which we should find evidence of specific mammal groups," Donoghue told Discovery News.

Michael Benton, a professor in the School of Earth Sciences at the University of Bristol, said he believes that the DNA/molecular clock approach of estimating an animal group's age, used by dos Reis and colleagues, "applies standard, accepted, conservative approaches that take account of missing data in the fossil record."

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

“The relevance to humans is that the placental ancestor is one of our ancestors," Donoghue said.

"It reveals the pattern of assembly of the (basic body plan) that we have inherited. As such, it allows us, for instance, to identify which animals may be best suited to biomedical research to better understand and mitigate congenital diseases."