Geology often determines biology. The supercontinent Pangaea, when all the world’s landmasses were joined together, began to break apart during the age of the dinosaurs. In the centre, a rift opened at the end of the Triassic between North America and West Africa, forming the very earliest beginnings of the Atlantic Ocean. During the Jurassic, the Tethys Ocean emerged between Laurasia (Eurasia plus North America) and Gondwana (the rest of the world) in the south. Think of the Tethys as the Mediterranean Sea, but stretched all the way into Asia. In the Cretaceous period, Africa and South America finally drifted apart, creating the South Atlantic Ocean.

More oceans meant more coastline, less drought, and fewer desert-like landmasses than had existed during the Permian and Triassic. Flowering plants, or angiosperms, benefited enormously from these changes. During the Cretaceous they began to outcompete the older world of ferns, horsetails and gymnosperms, such as ginkgos and conifers. This was gefundenes Fressen for groups such as the multituberculates, those highly successful archaic mammals.

Multituberculates looked like mice or rats and moved and fed in similar ways, yet they were not rodents. Rodents evolved much later, after the dinosaurs, within the "modern" placental mammals. Multituberculates were grinding machines and champions at processing plant matter. Leaves, stems, flowers, buds, fruits, and roots all ended up on their menu. Their success was due in part to the large number of little cusps ("multi" and "tubercula") on their molars.

A multituberculate molar resembles a Lego brick. Multituberculates possessed a differentiated, or multifunctional, dentition. Three middle-ear bones are distinctive of true mammals, but so is a Swiss Army knife-like set of teeth. Incisors were used to bring food into the mouth or gnaw it off. Canines ripped or cracked food into pieces. The lower premolars moved up and down against those of the upper jaw, crushing and slicing the food. And, last but not least, everything was ground into a pulp by a rubbing motion between the cusped upper and lower molars.

By processing food this way, mammals digest it more rapidly, which is essential because maintaining an internal furnace consumes large amounts of energy. By processing food this way, mammals digest it more rapidly, which is essential because maintaining an internal furnace consumes large amounts of energy.

This was a major difference from reptiles. While mammals process food extensively in the mouth, many reptiles use their teeth mainly as gripping tools and swallow food soon afterward. Reptiles continually replace their teeth throughout life. Mammals, by contrast, must make do with only two sets: milk teeth and a permanent adult dentition. That is the price of a sophisticated chewing apparatus. Efficient cutting and grinding would have been impossible if the teeth had constantly been in different stages of growth.

Multituberculates were not the only successful group of mammals to arise during the age of dinosaurs. There were also early versions of the placental mammals (eutherians, our own group) and marsupials (metatherians), together known as the Theria. In addition, there were the monotremes, egg-laying animals that also produced milk, such as today’s platypus.

The molars of these ancestors of modern mammals were more sophisticated than the Lego-brick-like teeth of the multituberculates. They featured miniature peaks and valleys. The cusps of the upper molars fitted into the basins of the lower molars.

These so-called ‘tribosphenic’ molars performed two tasks in a single movement: tearing and grinding. Imagine pressing a pestle into a mortar while making pesto. Multituberculates still required two different types of teeth, premolars and molars, to achieve the same result.

A small modification in a molar made a world of difference, and modern mammals, including ourselves, owe much of their success and existence to it. The Theria - so placentals and marsupials - became far more capable of exploiting the rapidly expanding world of insects. Insects, important pollinators, diversified alongside flowering plants. The mammals, with their new teeth, capable of both tearing and grinding, could more easily access the nutritious parts of insects.

The peaks and valleys of such molars were also highly adaptable. Small changes in their shape allowed mammals to exploit new food sources when environments changed. Carnivores, such as your cat or dog, have sharper cusps on their molars that function like scissors for slicing meat. If you run your tongue over your own molars, you can feel that the cusps are less pronounced and the chewing surface broader, making them better suited to our omnivorous diet.

Hundreds of species from these early successful mammal groups, including multituberculates, monotremes, marsupials, and placentals, would witness the fateful day 66 million years ago. Researchers have even determined that it was a spring day (in the Northern Hemisphere), based on growth lines preserved in the fossilized bones of fish that died that very day.

The story is well known. An asteroid roughly the size of Brussels struck the Yucatán Peninsula in Mexico, creating a crater roughly the size of Belgium. The impact triggered wildfires, earthquakes, tsunamis, and a persistent dust cloud that cooled the planet, halted photosynthesis, and wiped out three-quarters of all life in a geological instant. Among the dinosaurs, only a handful of bird species survived.

What is less well known is that nine out of every ten mammal species also perished. Mammals came frighteningly close to disappearing entirely. Every mammal alive after the catastrophe, including ourselves, owes its existence to a handful of survivors that crawled through the dust. They were probably small, omnivorous, and/or burrowing animals.

After 150 million years spent in the shadow of reptiles, it was finally time for mammals to shine. Mammals took over many of the ecological niches long occupied by triceratops, hadrosaurs, and T. rex, and they began to increase in size.

“Just 400,000 years after the asteroid impact, during the Paleocene, there were already 50-kilogram mammals the size of wolves, such as the carnivore Eoconodon,” says Thierry Smith. “Toward the end of the Paleocene, large herbivores appeared, such as the 500-kilogram Barylambda, about the size of a pony. This evolution went hand in hand with that of plants. Climate warming made flowering plants more diverse, including palms, nut trees, and legumes, all rich in energy.”

Placental mammals, in particular, grew larger. Their reproductive strategy offered advantages. Unlike marsupials and monotremes, the embryo develops inside the mother's body, nourished and protected by the placenta until it is born as a relatively large and well-developed young. Yet mammals were not the only survivors to produce giants during the ten million years after the apocalypse. There was Titanoboa, a snake up to thirteen meters long and weighing around a ton, and Gastornis, a two-meter-tall flightless herbivorous bird.

Although mammals became larger after the extinction of the dinosaurs, they did not immediately become smarter. Their brains remained relatively small in proportion to their body size. Perhaps they simply did not need large brains because so many ecological niches were available to them. What they clearly did invest in was movement. New ankle joints evolved in a wide variety of forms, enabling species to run, dig, and climb more effectively. They adapted to a remarkable diversity of environments and food sources.

The world was already warm in the aftermath of the dinosaurs’ extinction, but around 56 million years ago an additional and very abrupt episode of global warming occurred. The likely cause was the opening and expansion of the northern Atlantic Ocean. Volcanic activity along the Mid-Atlantic Ridge gradually separated North America and Greenland from Eurasia, a process that is still continuing today. The enormous quantities of CO₂ released into the atmosphere caused global temperatures to rise by another 5 to 8 degrees Celsius within just a few thousand years. Average temperatures reached around 34°C worldwide, roughly 40°C at the equator, and 25°C in the Arctic. Where there is now an icy Arctic Ocean, palm trees once grew. Palaeontologists call this episode the PETM, the Paleocene-Eocene Thermal Maximum.

“And this extremely hot world did not lead so much to extinction,” says Thierry Smith as we continue walking along the sunken lane in Dormaal, “but instead gave rise to a whole series of ‘modern’ placental mammals: primates, bats, carnivorous mammals, even-toed and odd-toed ungulates, rodents... They benefited from the warming and spread rapidly around the Northern Hemisphere via forested land bridges that connected Eurasia and North America.”

But where did this dynamic new generation suddenly come from? What was their common ancestor? “After 150 years of research, it remains a mystery. Personally, I think we should look for their cradle around India, back when that great landmass was still drifting toward Asia and had not yet created the Himalayas. We find the same species there as in Western Europe. Modern mammals may well have originated on the floating ‘island’ of India and migrated into Asia and Europe via temporary land bridges.”

Whatever their ultimate birthplace, it is in the gullies along this very sunken lane in Dormaal that palaeontologists have found some of the earliest modern mammals. During the nineteenth and twentieth centuries, an eighty-centimeter-thick fossil-bearing layer was exhaustively searched by washing sandy sediments through fine-meshed sieves. In the late 1980s and early 1990s, citizen palaeontologist Richard Smith wet-sieved no fewer than 23 tons of sediment.

This coarse material had been washed from higher ground into a nearby sea some 56 million years ago, accumulating in a bend of a river near Dormaal. River bends also act as traps for dead organisms. The deposits revealed plant remains: spores, pollen, and pieces of wood. These showed that the riverbanks were densely vegetated, including plants related to pines, walnuts, and poplars. Grapevines climbed their trunks.

Nearby, around modern-day Hoegaarden, there were marshlands resembling those of the Louisiana delta around New Orleans, complete with swamp cypresses standing knee-deep in water. During excavations for the E40 motorway in 1970 and the high-speed railway in 2000, dozens of fossilized tree trunks were unearthed. Some can still be seen in situ at the Goudberg geosite near Hoegaarden.

“Belgium was truly subtropical at the time: warm and humid,” says Thierry. In the Dormaal paleo-river swam crocodiles, alligators, giant turtles, and fish, especially pike. The riverbanks and forests buzzed and chirped with life, populated by frogs, salamanders, geckos, birds, and the first representatives of virtually every mammal group alive today.

Most of the fossil discoveries consist of teeth, more than 14,000 of them, because teeth are composed of enamel, a tissue that is 96 percent mineralized and fossilizes far better than the rest of the skeleton. “From the number of teeth, the spacing between them, their shape, and other features, we can quickly determine whether an animal was a predator, scavenger, insectivore, omnivore, or grazer. The unique pattern of ridges and cusps acts like an identity card and tells us exactly which species it belonged to.” The mammals of Dormaal represent about fifty taxa, mostly small species such as rodents, opossums, and small ungulates.

Thierry himself switched careers from marine biology to palaeontology through studying the mammal fossils discovered by his father. He had grown up immersed in the field because Richard Smith chose family vacation destinations based on nearby excavation sites. “As a teenager, I eventually got a little tired of that,” Thierry laughs. In 2014, father and son, together with colleagues, described Dormaalocyon latouri, 'the dog from Dormaal’. “It is one of the earliest ancestors of today’s carnivore mammals. In other words, a primitive predator that lived long before carnivores split into the more dog-like lineages, such as wolves, bears, martens, and seals, and the more cat-like lineages, including cats, tigers, and lions.”

What did this ancestor of your barking Luna or purring Felix look like? Something like a marten. It weighed only half a kilogram to one kilogram. From the structure of its flexible ankle joint, Thierry and his colleagues concluded that Dormaalocyon lived in trees and probably moved through the forest canopy. Long canines were already present, characteristic of modern cats and dogs, and it may have been omnivorous. It certainly ate insects in order to have enough proteins, but it may also have preyed upon even smaller mammals.

One of those prey species may have been its contemporary and fellow tree-dweller, Teilhardina belgica. Teilhardina is a crucial discovery because it represents our most distant known primate ancestor. The species was first described in 1927 by the famous French Jesuit priest and palaeontologist Pierre Teilhard de Chardin. Teilhard de Chardin was something of a thorn in the Church’s side because he attempted to reconcile evolutionary theory with faith and morality. The Vatican eventually prohibited him from publishing or lecturing about some of his ideas.

He described this earliest primate ancestor from a few jaw fragments and isolated teeth, initially naming it Omomys belgicus, a species later renamed Teilhardina belgica. “After Teilhard de Chardin, around three hundred additional fossils of the little primate were excavated, including limb bones, giving us a much clearer idea of what our distant primate ancestor looked like,” says Thierry. “It most closely resembled the modern tarsier or mouse lemur: extremely small, with large eyes, excellent grasping hands and feet, long fingers, flexible elbows, and a powerful leap.”

Primates possess several features that distinguish them from other mammals: forward-facing eyes with stereoscopic vision and depth perception, opposable thumbs and big toes, nails instead of claws on both hands and feet. “Every one of these traits is an adaptation for climbing trees more effectively.” The story of humanity, in a sense, begins among the branches of those subtropical forests of Dormaal.