Did Humans Really Evolve From Rats?

A recent study, published in Science magazine this month, hypothesizes on the appearance and characteristics of the mommy of all placenta-bearing mammals. The aptly named “hypothetical placental ancestor” (see image).

Placental mammals, of which humans are one such brand, are a very diverse group of species bearing live young. Young that are nourished before birth in the mother's uterus through a specialized embryonic organ, called the placenta.

Today, over 5,000 placental mammals exist on Earth, ranging in size from 1.5 g (the bumblebee bat) to 190,000 kg (the blue whale), differing in locomotive ability (from running to flying to swimming creatures) and with very different brain size to body ratios. Therefore, pinpointing a common ancestor for all placenta-forming species seems like an insurmountable task.

Scientists believed that these placenta-bearing creatures came about ~66 million years ago either just before or just after a massive extinction event that led to the elimination of around 75% of animals and plants, and more famously to the downfall of the dinosaur empire. This Cretaceous-Paleogene (K-Pg) boundary rolled (albeit violently) into the Paleogenic period: picture a radically transformed global ecosystem, where anything that survived would have had to deal with a vastly altered climate and diet.

Through painstaking analysis and the inputting of nearly 400,000 observation points, by a team of 23 researchers from six different countries, this work compiled the intricate external features of 86 living species and 40 fossils of extinct animals in one cloud based application (Morphobank). In a massive global effort to create a “super-spreadsheet,” they were able to use comparative analysis of features like diet, length of limbs, shapes and patterns of teeth, as well as images of fossils and living species, to show us the most likely candidate for this 66 million year old mammal; for which there is no known fossil record. This insect-eating rodent-type climbed trees, it weighed less than half a pound, and gave birth to blind, hairless babies. And with all their collective data, from fossil records and current species, this team of scientists was also able to suppose that our snuffling predecessor came about after that extinction event and not before; a hotly debated topic in the past.

Our rummaging forerunner lived a mindboggling long time ago, but now with a catchy name and a color image highlighting her main glorious features we have finally been introduced to our ancient predecessor. Let’s take a moment now to marvel at evolution (pause for quiet time).

For more information check out the primary author, Dr. Maureen O’Leary of Stony Brook University, chatting about “HPA” in this American Museum of Natural History video.

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Beth Ashbridge

PhD in Chemical Biology. Currently Research Fellow in Cancer Biology (www.bettybooashbridge.wordpress.com)

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