If you are not familiar with the Miocene, you should be. This is when your great, great, times 800,000, grand daddy was picking his nose and thinking long and hard about your grandma’s charms. It was a time characterized by strange weather, cold and warm then cold again and a new animal arrived on the block; apes and we know how that turned out.
It started 23 million years ago, which might seem like a long way in the past, but think of the earth as a person that lived to 100 years. When the Miocene began, this ‘person’ would have been ninety five years old. Everything that has occurred since then has been in the last 5 years.
The Miocene ran from 23 million years ago to 5 million years ago. Lots of interesting things happened just before this too, including in Australia, the world’s favourite koala evolved (OK it’s the world’s only koala) from the wombat which digs a burrow. This makes life a bit difficult for the koala because the pouch faces the right way for digging but the wrong way for climbing trees, which was a surprise to mummy koalas by all accounts.
Not sure how many little koalas fell out on the way to mum’s pouch before they figured it out, but once there, she zips up the bottom until the little pink jelly bean latches onto the teat which swells up in his mouth so he has something to hang onto to stop falling out the bottom.
Over in Africa the final touches were being put on the world famous Rift Valley and one animal split into two types, monkey and ape. While this was going on, an asteroid paid us a visit and punched a rather substantial hole into Tajikistan.
The apes meanwhile spread out and developed into 100 species, which included a couple we recognize today, the chimp and the bonobo. (We don’t know for sure which one or several ape species contributed to the animal from which we descended, but molecular evidence strongly suggest it lived 7 to 8 million years ago. The mobile-on-two-legs version evolved in Africa later, about 5 million years ago. This is also when the chimpanzee–human divergence occurred.)
The overall climate trend was ‘cooling-ever-so-slowly’ but not long after starting, it warmed up for a short time, about 6 million years, which is not very long if you are 4,600 million years old.
For the period we are interested in, here at the 20 million year mark, it was warm and still moist, supporting vast forests, but that was to change. The plants and animals were fairly modern versions many of which we would recognize today. The mammals and the birds were well-established right across the planet.
In the seas, kelp forests made their first appearance and soon became one of Earth’s most productive ecosystems. Seals, whales and others helped to spread kelp beds around the globe.
Continents continued to drift as they always have, toward their present positions. Only the land bridge between South and North America was absent from what we see today, although many land masses have moved further into their present position. For example, Australia was about 1200 kilometres (720 miles) further south. The whole continent moves north about 6 metres (20 foot) in a human life time.
As for most of earth’s history, mountain building was going on relentlessly, just as erosion was tearing them down. India continued its determined drive into China driving up the Himalayas, just as it does today.
The most noticeable change to the land was the formation of the grasslands and the slow replacement of trees in many area as the climate dried out. Grazers, like the horse, rhinoceroses, and the hippos expanded their numbers as more land opened up for them.
As the grasses evolved to be fibrous and fire-tolerant, long-legged grazers with high-crowned teeth, that is those with enamel covering the whole tooth down to below the gum line, expanded their numbers into the millions with rapidly developing predators following across vast open plains.
This was a golden age for marine birds and there has never been more diversity than there was 20 million years ago. Land based birds found this period to their liking too and by the end of the Miocene just about all modern birds were present. At ground level, some of the animals that had been around for millions of years were still plentiful, like sabre-tooth ‘cats’ (not strictly true cats) two metre high pigs (now extinct fortunately) another more friendly pig-like herbivore and small, three-toed horses.
Ice had been building up in Antarctica for the last 15 million years, especially in the form of glaciers but any expansion was halted by this brief warm period. When Australia left the Antarctic super continent, the cool currents encircled the continent and eventually froze it solid but this also had an effect on other oceans. In fact the Greenland icecap was eventually to form as a result of ocean cooling generally.
So, it was not really all that long to wait before our group the apes and our distant cousins the monkeys were both to re-develop 3 colour vision (yes, it was invented already) which was very useful for detecting ripe fruit. The moon, which is still moving away from us, was about 50% bigger than it appears in our night sky tonight. Not long after that, there was a massive supervolvano eruption in North America, a place we know today as Yellowstone National Park, which made rather a big mess all over the planet and that took a few years to clean up.
Hardly any time later, our little club that includes chimps, gorillas, humans and orangutans split from gibbons who have remained pretty much unchanged since the divorce.
But something was on the horizon, a silent change, that was to lay the foundations for modern humans. 15 million years ago, the Pacific and Caribbean tectonic plates gently collided forming the Isthmus of Panama, which critically changed ocean currents, drying out Africa, reducing the forests and forcing one group of apes onto the plains.
Soon we would split into the four groups of ‘great apes’, gorillas, chimps/bonobos, orangutans and humans.
References and further reading
At the very beginning: The Big Bang
How to build a star
The Miocene
The Whole History of the Ape Family