The short version: in the beginning, there was nothing, which then exploded.
The longer version: all the matter in the universe was compressed into the smallest possible volume. Try to understand that this is so much matter that the force of gravity warps the laws of physics as we know them. The whole thing is is under so much pressure that it explodes – forming the universe as we know it as the laws of physics change radically from picosecond to picosecond, and eventually energy cools and congeals into matter.
It’s like they say: “it all started with the Big Bang!”
In the early millenia of what is sometimes referred to as the Hadean era of the Earth, there were no rocks as we would commonly understand the term – it was too hot for them to form. Still, the Earth was slowly cooling and solidifying. It’s worth noting that the Sun itself was not as hot at this time – like the larger planets of our solar system, it was still accreting matter to itself. Rockballs like the Earth (and Mars and Venus) were largely done with this process (although the occasional meteor or cometary impacts still occurred).
By the end of this era, approximately 3,800,000,000 years ago, the Earth had cooled sufficiently to allow for the stable formation of rocks, and its surface had begun to split into tectonic plates. Most importantly for humanity’s future, life had begun: the earliest evidence of photosynthesis dates from around this time.
The majestic oceans of planet Earth were formed neither quickly nor simply. It took literally millions of years between the first surface water’s appearance and the creation of the primordial sea.
Several factors contributed to this: the gradual cooling of the Earth was the first and most important, but also important was the slow release of water from existing minerals, the condensation of steam, and even the addition of water in the form of ice from occasional cometary collisions with the planet.
The first waters soon became the habitat of early prokaryotes – whose biochemical processes led to the formation of still more water. Indeed, it is possible that the majority of water on the planet today exists as a result of these organisms.
The autotrophs are the first true organisms on the planet Earth. The very first of them, the first prokaryotic life on earth, were chemotrophs. Chemotrophic organisms utilise reactions with inorganic chemicals to generate the energy they need to survive. They were followed by other autoptrophs – lithotrophs, a more specialised form that rely on minerals, and phototrophs, the earliest photosynthetic organisms.
These organisms were the common origin of all life on Earth, from single-celled bacteria that have not really changed that much in millions of years, to every multi-celled organism on the planet. Even you.
The Proterozoic Eon, which began some 2.5 billion years ago and ended about 542 million years ago, was traditionally regarded as the last era before the evolution of what we would recognise as animals. But more recent fossil discoveries have repainted this picture, and it now appears that animals roughly midway through the Ediacaran Period.
The earliest animals were very simple creatures- small invertebrate multi-cellular forms like the earliest worms and such. Arthropods evolved shortly thereafter, and would go on to become the most dominant type of animal on the planet – just think how many insects and spiders and such you see each day if you don’t believe that.
The earliest known fossil footprints on land actually predate the earliest definitively land-based animals fossils by a considerable margin: 170 million years. It appears that our early ancestors may have explored the land before they moved there permanently. In fact, fossil records suggest that this exploration began before there were even terrestrial (as opposed to aquatic) plants – which may account for this peculiarity: no plants would have meant no food.
The first known animals to leave the ocean for good were members of the superclass Tetrapoda, a large group that includes all amphibians, reptiles, birds, and mammals, living and extinct. Including us.
The earliest known bipedal vertebrate, eudibamus cursoris was a small parareptile. The sole specimen that has been found (in Thuringia, Germany) measured about 25 cm long – about the size of a house cat. Reconstructions of it give it an appearance resembling a cross between a tiny velociraptor and a modern iguana.
The sole specimen of it known to science was discovered in 2000 by a paleontological team including David S. Berman, Robert R. Reisz, Diane Scott, Amy C. Henrici, Stuart S. Sumida and Thomas Martens. The species is believed to have existed for a span of about five million years or so.
Pangaea was a super-continent – an agglomeration of multiple continents – that came into being about 250 million years ago. It was composed of all the continents we know today fused into a single landmass, surrounded by a single ocean (called Panthalassa) – and was the last time such a thing occurred. In fact, it was slightly larger than the combined areas of the modern continents, as supercontinent formation tends to lead to lower sea levels.
Pangaea (the name comes from the Greek Pan meaning All and Gaea meaning Earth) existed during the Paleozoic and Mesozoic eras, and its best known inhabitants were the dinosaurs. It began to break up approximately 75 million years after it formed, although the continents would not reach anything approximating their modern positions until only about 35 million years ago, when the Indian subcontinent collided with Asia.
There remains some question which dinosaur was actually the earliest of its kind. The oldest claimant is of the order Saurischia (literally ‘lizard-hipped’ dinosaurs), the Nyasasaurus, dated at 243 million years ago. The next oldest saurischians are dated between 232 and 230 millions years ago.
The earliest true dinosaurs of the Ornithischia order (the other major order of the Dinosauria clade) are likewise dated at 230 to 220 million years ago, the oldest of which is the Pisanosaurus.
Everyone loves the dinosaurs. A lot of people – if the Jurassic Park films are to be believed – would like to see them come back. But without their extinction, we wouldn’t be here today.
Even now, it’s still not clear what exactly caused the extinction event – but the best known hypothesis is that of Luis and Walter Alvarez, which states that a meteoric or cometary impact caused a nuclear winter-like effect that altered the climate drastically, wiping out something like 75% of all species alive at the time. The effects were particularly felt by larger species – which included most dinosaurs.
In the wake of the event, now open evolutionary niches were occupied by mammals and birds, including our own ancestors.