66 million years ago, an enormous bolide meteor impact in what is now Mexico’s Yucatan Peninsula sealed the fate of more than three-quarters of Earth’s plant and animal species. In the devastating decades-long global winter that followed, the larger dinosaurs were the first to die out, followed soon after by most of their relatives.
Where dinosaurs, the ancestors of all modern birds, once dominated the land, many niches opened up for new species to develop.
Mammals, tiny versions of which had coexisted with the dinosaurs for some 150 million years, took this opportunity to evolve from their shrew-like ancestors to become Earth’s new rulers.
Table of contents
1. Phenacodus
Horses have been domesticated for around 4,000 years, but their ancestors started arriving on the scene only a few million years after the dinosaurs disappeared.
One of the earliest ancestors of all modern equines is the phenacodus, which lived around 55 million years ago in what is now North America.
Phenacodus was a member of a family that comprised many similar creatures spanning a wide range of body sizes from that of a domestic cat to a goat.
Phenacodus is generally considered to have been an herbivore, like modern horses, and is also likely the common ancestor of rhinoceroses and tapirs.
2. Hesperocyon
Dogs were the first animal ever domesticated, but most people know that they’re still closely related to other canines, such as wolves and foxes. However, all canines share a common ancestor, and most evidence points to hesperocyon.
Hesperocyon also lived during the Eocene, when most of the families of current mammals came into being.
The animal was little more than two feet long, with an appearance similar to that of a raccoon. The fossil record suggests that hesperocyon split off from the two other suborders of caniforms during the Eocene, those being the extinct amphicyonidae ‘bear dogs’ and the arctoidea, which includes beavers and bears.
3. Proailurus
During the period following the Eocene, mammals started to diversify even more, and it was at this time that felines came into being.
Proailurus is perhaps the earliest ancestor of all cats including panthers, lions, tigers, and, of course, your household moggy.
Proailurus, meaning ‘first cat,’ would be easily recognizable to this day as a feline, even though it lived some 25 million years ago.
The proailurus was about two feet long and weighed 20 pounds and, like all felines, they were carnivorous. Evidence also points to them being largely arboreal, waiting in trees to hunt prey. Their habitat comprised huge swathes of Eurasia, favoring heavily forested regions.
4. Dormaalocyon
55 million years ago, lived the ancestor to all modern-day carnivorous mammals.
Dormaalocyon was one of the earliest carnivoraforms, the order of animals that all canines and felines belong to.
The creature looked more like a squirrel than either a cat or a dog, but evidence strongly points to it being an ancestor of both.
The dormaalocyon takes its name from the village of Dormaal in Belgium where it was discovered in 2014. Studies of its teeth prove that it was a carnivore, and it probably preyed on both smaller mammals and insects. However, the dormaalocyon was very small, weighing only around 2 pounds.
5. Diacodexis
Pigs, camels, deer, cows, and even whales are just a few of the major species that make up the order artiodactyla (even-toed ungulates), meaning that they all have a common ancestor.
The earliest known member of the order, and a common ancestor to all of the above, is the diacodexis, a small herbivore that lived from around 55 to 46 million years ago during the Eocene.
The fossil record suggests that the creature looked similar to a very small modern-day antelope, measuring only 1.6 feet in body length.
Around 46 million years ago, diacodexis evolved to branch off into three main subgroups of even-toed ungulates, including suidae (pig, etc.) and hippos.
6. Purgatorius
If the history of Earth was a 12-hour clock, humans have been around for 77 seconds
If the history of Earth was a 12-hour clock, humans have been around for 77 seconds, so they’re rather new on the scene compared to many other species.
However, humans belong to the order of primates, which can trace its routes as far back as 66 million years ago, which was also the time of the Cretaceous-Paleogene extinction event.
Purgatorius is now widely believed to be the earliest primate, though it is sometimes referred to as a proto-primate as well.
Purgatorius looked far more like a mouse than anything resembling a modern primate, and it was likely an agile tree climber.
7. Eotragus
Sheep, goats, cows, and many other species make up the taxonomic family bovidae, one of the main extant families of the artiodactyla order.
Again, they all share a common ancestor, one of which may be the eotragus.
Eotragus lived around 20 million years ago during the Miocene, long after diacodexis had diversified into multiple distinct groups. Few remains have ever been found of this ancient bovine, although evidence points to it being similar in appearance to a small antelope.
Eotragus and all bovines evolved separately from other members of the artiodactyla order after crossing from North America to Eurasia when both were connected.
8. Palaeolagus
Rabbits, hares, and pikas (small rodent-like creatures) form the largest modern members of the order lagomorpha and thus share a common ancestor.
This creature might have been the paleolagus, a long-extinct genus of lagomorph that lived between 33 and 23 million years ago.
It was about the same size as modern rabbits but, due to what is known about its bone structure, it likely moved in a very different manner. Few fossils or skeletal remains have ever been found, which is a common problem among many smaller mammals with weaker bone structures that don’t survive through the eons as well.
Final Thoughts
Although all of the above appear to be direct descendants to common animals found today, they also had many relatives that branched off to form groups of their own.
Many of these groups, such as with once common animals such as the amphicyonidae (bear dog), moeritherium (related to pigs and elephants) have not survived to this day.
Humans share this planet with the ancestors of many of these early species. Our time on earth has been relatively short-lived: a fraction of time in the evolution of our planet.