The concept of a "Multicellular organism" is a surprisingly vague one. Single cell organisms have grouped into colonies in order to increase both individual and collective survival since three billion years ago, the algae like "Cyanobacteria" possibly being the first such species to do such (also being one of the first to produce oxygen). The first true single creature to be composed of multiple cells however, with permanently connected parts specialized in specific ways to promote survival appeared about one billion years ago. A likely candidate for this first creature to move beyond a mere colony to a single being is the red algae Bangiomorpha pubescens, its differentiated cells helping with attachment as well as making the red algae the first known organism to exhibit sexual reproduction. After this advancement multicellular structures would evolve independently across many differing species.
Sexual reproduction is a form of reproduction that, unlike asexual reproduction like mitosis, involved multiple partners. Typically, specialized cells known as "Gametes" (such and sperm and eggs) will combine to make a Zygote, which will grow into a new member of the species. The advantage of sexual reproduction over asexual is a complicated one, while something like mitosis would allow for any member of a species to reproduce with more freedom it means doing so with greatly less genetic diversity with a higher chance of maintaining bad mutations. These factors have led to the vast majority of Eukaryotic species to reproduce sexually.
Eukaryogenesis is the process by which prokaryotes became eukaryotes. Early organisms were all prokaryotes, incredibly simple, (at least when compared to modern organisms) single celled beings lacking most organelles, only having the most basic systems to maintain homeostasis and reproduce. Eukaryotes refer to organisms whose cells have complicated internal structures beyond the bare necessity to stay alive and procreate, particularly a membrane-bound nucleus. The creation of these cells is still somewhat disputed, but the most likely theory is that of Symbiogenesis. That a "host cell" engulfed a bacterium in its entirety, and instead of ingesting it, the cell and bacteria formed a stable, symbiotic relationship, one maintained after mitosis. This development is what laid the groundwork for the later evolution of multi-cellular organisms.
The first mammals evolved roughly 225 million years ago, during the late Triassic period. Named mammals for the milk producing mammary gland, also usually (but not always) practicing live birth and having a form of fur or hair. The first mammal was the morganucodontids, a small nocturnal insectivore, like most early mammals stuck in the shadow of dinosaurs. They evolved from synapsid ancestors, unable to survive by raw power over millions of years of evolutionary pressure they developed features centering on the more practical aspects of survival, differentiated teeth (incisors, canines, molars) and a secondary palate to breathe while chewing, warm bloodedness to maintain body heat combined with insulating fur. After the eventual extinction of the dinosaur, they were already prepared to inherit the earth.
In the late Triassic period, approximately 20 million years after the great dying led to the extinction of most life on Earth a new clade of creature began to claim a place of ecological dominance, Dinosauria. Literally named "terrible reptiles" after the discovery of their fossils in the mid 1800s. The group, taxonomically speaking includes several clades of typically large warm-blooded reptiles, along with modern birds. Though not technically a part of the scientific definition they colloquially include Pterosaur and Mosasaur, flying and aquatic giant reptiles that existed around the same time period. They would come to define the Mesozoic era before ending their dominance as it began, with a mass extinction event.
In the wake of the development of photosynthesis there was a small yet ecologically significant increase in the amount of atmospheric oxygen, named the "Great Oxidation Event." Over the course of the next four hundred million years this led to a great decline in the number of anaerobic organisms and set the stage for aerobic metabolism and later eukaryotic evolution. Oxygen in the air also likely led to the oxygenation of strong greenhouse gases into weaker ones, leading to a sort of global cooling effect.
For hundreds of millions of years, life, marine and terrestrial had evolved on Earth, then, over just one to two hundred thousand years upwards of 90% would be gone. The Permian–Triassic Extinction Event, more commonly known as the "Great Dying" wiped out the majority of earth's biological diversity marking the end of the Permian era. While the exact events are debated most likely a series of volcanic eruptions in what is now Siberia led to a mass increase in atmospheric carbon dioxide, both increasing global temperatures by possibly more than 14 degrees Fahrenheit; but also leading to acid rain and ocean acidification. The vast majority of plant and animal species, especially marine life, were entirely unprepared for such an event. Unable to adapt, they simply went extinct. With the elimination of many of the creatures that had dominated ecosystems for millennia, such as the famous Trilobite, having surviving since the Cambrian era, there was something of an ecological power vacuum left over. Across the millions of years of recovery creatures such as mammals and the dinosaur began to take their place.
Homo Sapiens in their modern form emerged around 300,000 years ago in East Africa, having evolved from and interbred with other species of great ape in the genus homo. Defined by bipedalism, increased brain size and sophisticated fine motor movement they would go on to become the dominate species on Earth, after migrating beyond East Africa they would go on to have a permeant pretense on every Continent on Earth, along with space. Their high intellect allowed for the creation of complex Eusocial and widely varied organizational structures, the history of these can be learned about on the other timelines on this website.
Photosynthesis is the process by which plants convert light energy into a useable form. Most modern photosynthesis involves using six Carbon Dioxide molecules and six water molecules to create a molecule of glucose and six molecules of oxygen. That glucose molecule can then be broken down to create ATP or energy for the cell. It happens in two stages, the "light-dependent reaction" where light is captured in specialized organelles called "Chloroplasts" and its energy is used to split water in order to create the materials used to convert carbon dioxide into glucose in the second phase or "light independent reaction." The excesses oxygen created in this process being expelled into the atmosphere. This provides the base layer of every food chain and ecosystem on earth, bar some deep-sea vents, without photosynthesis the vast majority of energy used by life would not exist. The earliest examples of photosynthesis were anoxygenic, typically using other hydrogen-based substances such as Hydrogen sulfide or Hydrogen gas instead of water thus not producing excess oxygen. Over time this would change allowing for the great oxygenation event.
Tetrapods, literally meaning "four foot" in Greek, refer to all four limbed animals and the creatures descended from them, even if they do not currently have four limbs. This very eclectic group includes reptiles, birds and mammals. They evolved from lobe‑finned fish (fishes with fleshy, lobed fins containing bones and muscles) during the Devonian period or the "age of fishes" defined by large shallow seas lush with vegetation. These environmental pressures favored strong limbs and lungs over fins or gills. Over the course of millions of years of evolution creatures like the Tiktaalik would develop. These early "fishapods" had features such as fins resembling limbs and strong rib cages, allowing them to better support their bodies and eventually, they started breathing air; Slowly becoming early tetrapods. The first of the clade still remained aquatic, but these innovations laid the groundwork for future land-based animals.
