Life on the planet started astonishingly early. The first living organisms, in the current model of evolution, are thought to be Prokaryotes1. The oldest known fossilised prokaryotes have been dated to approximately 3.5 billion years ago, only 1 billion years after the formation of the Earth's crust.
Eukaryotes2 are more advanced organisms with complex cell structures, each of which contains a nucleus. Although incredibly hard to determine their origin, they are thought to have developed 1.6–2.1 billion years ago, although some research2 suggests eukaryotes being present even earlier than this.
Around 1.1 billion years ago multicellular3 organisms are thought to have started to develop, most likely similar in form to plants such as green algae. 200 million years later true multicellularity had also evolved in animals similar in nature to today's sponges, which are organisms which can reassemble themselves.
Animals4, in the most basic sense of the word, are considered to have evolved from Eukaryotes. Fossils of early sponges have been discovered in 665 million year old rock. Later on, around 560 million years ago, some highly significant fossils of an organism which was named Charnia4.1 were discovered. These enigmatic early animals were anchored to the sea floor where they are thought to have absorbed nutrients.
Around 450 million years ago, during the Ordovician Period, land plants appeared, although new evidence may suggest that complex photosynthetic plants developed over 1000 million years ago. Studies of fossils from the Devonian Period 416–359 Ma5.1 indicate that land plants had evolved features we recognise today, such as leaves, roots, and secondary wood. Towards the end of this time seeds had evolved.
The dominance of the Dinosaurs6 lasted for over 160 million years, from around 230 Ma, to their ultimate demise at the end of the Cretaceous 65 million years ago. The extinction of most dinosaur species occurred during the Cretaceous–Tertiary extinction event6.1. The fossil record indicates that birds evolved within theropod dinosaurs during the Jurassic period. Some of them survived the Cretaceous–Tertiary extinction event, including the ancestors of all modern birds.
The first Mammals7 are our most direct ancestors, they evolved from Amniotes7.1 which were a group of tetrapod vertebrates (four-limbed animals with backbones or spinal columns). All mammals posses the same characteristics; they are warm-blooded vertebrate animals of a class that is distinguished by the possession of hair or fur, the secretion of milk by females, and (typically) the birth of live young. One of the earliest known mammals was Eozostrodon7.2 which lived during the late Triassic and the early Jurassic, about 210 million years ago.
The genus Homo gave rise to modern humans8, Homo sapiens, us. It is estimated we have been around for 2.3–2.4 million years, coinciding with the first evidence of stone tool usage. Incredibly, recent evidence from Ethiopia8 places the earliest signs of stone tool usage at before 3.39 million years ago.
Modern humans have evolved into highly intelligent beings who posses the power, and desire, to understand who we are, where we came from, and how the natural world works. We have even become ambitious enough to try and help each other to understand these amazing things through ever more advancing visual communication!