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Earliest forms of life

4,100,000,000 (4.1 billion years ago)

The first microbial life on Earth according to biologically fractionated graphite inside a single zircon grain in the Jack Hills range of Australia. Potentially biogenic carbon isotope ratios have been identified for graphite embedded within a zircon from the site.

3,800,000,000

First living organisms on earth, first microbial life emerges in earth’s oceans

The earliest life forms we know of were microscopic organisms (microbes) that left signals of their presence in rocks. The signals consisted of a type of carbon molecule that is produced by living things.

Evidence of microbes was also preserved in the hard structures (“stromatolites”) they made. Stromatolites are created as sticky mats of microbe’s trap and bind sediments into layers. Minerals precipitate inside the layers, creating durable structures even as the microbes die off

2,400,000,000

Photosynthesizing bacteria evolved

Cyanobacteria, which developed oxygenic photosynthesis, emerged significantly impacting Earth's atmosphere by releasing oxygen into the atmosphere via photosynthesis and, in a few hundred million years, were able to change the composition of the atmosphere into what we have today. Our modern atmosphere is comprised of 78 percent nitrogen and 21 percent oxygen, among other gases, which enables it to support the many lives residing within it.

2,100,000,000

Eukaryotic cell evolved through a process called eukaryogenesis, which likely involved the merging of an archaeal cell with one or more bacteria in a symbiotic relationship. This led to the development of complex cellular structures, including a membrane-bound nucleus and organelles like mitochondria

1,200,000,000

Meiosis, a special type of cell division of germ cells in sexually reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilization, the haploid cells produced by meiosis from a male and a female will fuse to create a zygote, a cell with two copies of each chromosome.

In meiosis, the chromosomes duplicate (during interphase) and homologous chromosomes exchange genetic information (chromosomal crossover) during the first division, called meiosis I. The daughter cells divide again in meiosis II, splitting up sister chromatids to form haploid gametes. Two gametes fuse during fertilization, forming a diploid cell (zygote) with a complete set of paired chromosomes.

Meiosis occurs in all sexually reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants, and fungi. It is an essential process for oogenesis and spermatogenesis.

1,000,000,000

First non-marine eukaryotes move to land, they were photosynthetic and multicellular, organisms with complex cells that live in freshwater or terrestrial environments, as opposed to marine settings.

A multicellular organism is an organism that consists of more than one cell, and more than one cell type, unlike unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. Colonial organisms are the result of many identical individuals joining together to form a colony.

The origin of multicellularity is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit. This is essentially what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte. A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism is observable in Drosophila). A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues. This is what plant and animal embryos do as well as colonial choanoflagellates.

Because the first multicellular organisms were simple, soft organisms lacking bone, shell, or other hard body parts, they are not well preserved in the fossil record.

The colonial theory of Haeckel, 1874, proposes that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. At least some – it is presumed land-evolved – multicellularity occurs by cells separating and then rejoining (e.g., cellular slime molds) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division. The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation.

750,000,000

Beginning of animal evolution, multicellular, eukaryotic organisms comprising the biological kingdom Animalia. Animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals form a clade, meaning that they arose from a single common ancestor.

Over 1.5 million living animal species have been described, of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 0.00033 inches to 110 feet. They have complex ecologies and interactions with each other and their environments, forming intricate food webs.

Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa, and the larger, non-motile gametes are ova. These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm, also develops between them. These germ layers then differentiate from tissues and organs.

Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in Hydra and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.

The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to 200 tons and measuring up to 110 ft long. The largest extant terrestrial animal is the African bush elephant, weighing up to 13 tons and measuring up to 35 feet long. The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 80 tons, and Supersaurus which may have reached 125 feet. Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 micrometers and one of the smallest species (Myxobolus shekel) is no more than 8.5 micrometers when fully grown.