A. In 1980 a team of researchers were analysing soil samples at what was then known as the KT boundary. The K is misleading, as it actually refers to the cretaceous era, while the T refers to the tertiary era. What made geologists originally place a division in that distant time, some 65 million years ago, was the mass extinction which then occurred, seeing over two thirds of all land and sea life disappear, including the dinosaurs — or more strictly, all non-birdlike dinosaurs (since birds are now considered dinosaurs’ descendants). Whilst this was not the biggest extinction of all, it is definitely the most famous. But what caused it?

B. The researchers discovered that sedimentary layers at the KT boundary contained a concentration of iridium many times higher than what normally occurs — up to 120 times. Most iridium disappeared when the Earth was molten, sinking into its metallic core. However, this element is abundant in asteroids and comets, which led to an intriguing hypothesis — that an asteroid or comet had struck the Earth, causing the mass extinction.
The object would have vaporised almost immediately upon impact, throwing its iridium-rich contents into the atmosphere, from where it eventually settled across the planet. The problem was, an asteroid large enough to do this would have left traces of its impact in the Earth’s crust, and at that time there were no known signs. Or were there?

C. In actual fact, in the 1960s, a contractor named Baltosser working for a Mexican state-owned oil company had looked at a gravity map of the Yucatán Peninsula, near the Gulf of Mexico. He noticed a large arc-shape, showing a symmetry that was impossible to naturally occur. Company policy forbade him from releasing his findings, and so the secret lay until 1978, when two geophysicists, Camargo and Penfield, working for the same company, discovered it again.
In the search for possible oil-drilling sites, they had been examining magnetic surveys in the Gulf of Mexico, which revealed an underwater arc. The two arcs, seabased and land-based, matched perfectly, showing a circle 180 kilometers wide, centred on the coastal village of Chicxulub, and so it became known as the Chicxulub Crater.

D. In 1981, Camargo and Penfield released their findings, but the world was not listening. It took over ten years, and much more evidence (rock samples, drilling cores, and dating of the seabed rocks to the magic figure of 65 million years), before scientists began to accept the findings, although widespread skepticism existed, and still remains, to some extent, today.
It is occasionally argued that the impact was not the sole reason for the mass extinction, or that there were other contemporaneous impacts, or that extensive volcanism or climate and sea-level change were the real causes. It was perhaps this that led, in 2010, to an international panel of over 40 scientists being convened in order to specifically address the evidence. They concluded that an asteroid impact, as evidenced by the Chicxulub Crater, was indeed the cause of the mass extinction.

E. Trying to picture that event, the most powerful ever in the Earth’s history, strains the imagination. It begins with a 10–15 kilometer wide rock appearing from nowhere, almost instantaneously vaporising, and releasing over two million times the energy of an atomic bomb. The most immediate effect is a cloud of super-heated dust, ash, and steam expanding outwards, igniting fires, and broiling everything in its path.
A split second later follows a series of shock waves, traveling across the globe, triggering earthquakes and volcanic eruptions. Next there is a mega-tsunami, thousands of meters high, ripping coastlines apart and stirring up the oceans. Then, in the next few weeks, the huge amounts of carbon dioxide from the vaporisation of carbonate rock heats the Earth, but with the atmosphere choked with dust for years, sunlight is blocked, killing off plants, ultimately plunging the Earth into winter and the entire biosphere into absolute chaos.

F. The surprising fact is not that so many creatures became extinct, but that so many survived! With global disruption to plant communities, the herbivorous dinosaurs died quickly, and their predators soon followed. Sea-based life suffered disastrously, and all giant marine reptiles disappeared, yet the ancestors of the crocodile survived.
It is theorised that, like modern crocodiles, they were semi-aquatic and thus were able to shelter in water from fires and blast damage, and yet could scavenge on land amongst the abundance of dead animals for years afterwards. Similarly, insects, worms, and molluscs could all feed on dead plant and animal matter, allowing those that fed on these creatures to survive. Consequently, insectivores, scavengers, or omnivores, including mammals and smaller bird-like reptiles, were preserved.

G. Thus, the dinosaurs, as we know them, after 135 million years as the dominant land animal, were all but gone. This allowed mammals, then only small burrowing cat-like creatures (attributes which had also helped ensure their survival throughout the disaster), to emerge from the undergrowth, diversify, and eventually rule the land. In an ironic consequence, that class of animal ultimately led to species Homo sapiens, or human beings. So, were it not for that disastrous extinction 65 million years ago, we would not be here today.