Timeline of human evolution

3900 Ma

Prokaryotic Cell

Prokaryotic Cell

Prokaryotes Cell

This marks the first appearance of photosynthesis and therefore the first occurrence of large quantities of oxygen on the earth.

Prokaryotes are single-celled organisms that are the earliest and most primitive forms of life on earth. As organized in the Three Domain System, prokaryotes include bacteria and archaeans. Prokaryotes are able to live and thrive in various types of environments including extreme habitats such as hydrothermal vents, hot springs, swamps, wetlands, and the guts of animals.

Prokaryotic cells are not as complex as eukaryotic cells . They have no true nucleus as the DNA is not contained within a membrane or separated from the rest of the cell, but is coiled up in a region of the cytoplasm called the nucleoid. Using bacteria as our sample prokaryote, the following structures can be found in bacterial cells:

  • Capsule – Found in some bacterial cells, this additional outer covering protects the cell when it is engulfed by other organisms, assists in retaining moisture, and helps the cell adhere to surfaces and nutrients.
  • Cell Wall – Outer covering of most cells that protects the bacterial cell and gives it shape.
  • Cytoplasm – A gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.
  • Cell Membrane or Plasma Membrane – Surrounds the cell’s cytoplasm and regulates the flow of substances in and out of the cell.
  • Pili – Hair-like structures on the surface of the cell that attach to other bacterial cells. Shorter pili called fimbriae help bacteria attach to surfaces.
  • Flagella – Long, whip-like protrusion that aids in cellular locomotion.
  • Ribosomes – Cell structures responsible for protein production.
  • Plasmids – Gene carrying, circular DNA structures that are not involved in reproduction.
  • Nucleiod Region – Area of the cytoplasm that contains the single bacterial DNA molecule.

2500 Ma


First organisms to utilize oxygen

By 2400 Ma, in what is referred to as the Great Oxygenation Event, the pre-oxygen anaerobic forms of life were wiped out by the oxygen consumers.


2100 Ma



More complex cells appear.

A eukaryote is any organism whose cells contain a nucleus and other structures (organelles) enclosed within membranes. Eukaryotes are formally the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, enclosed by the nuclear envelope, which contains the genetic material. The presence of a nucleus gives eukaryotes their name, which comes from the Greek ευ (eu, “well”) and κάρυον (karyon, “nut” or “kernel”). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria or the Golgi apparatus. In addition, plants and algae contain chloroplasts. Many unicellular organisms are eukaryotes, such as protozoa. All multicellular organisms are eukaryotes, including animals, plants and fungi.

Cell division in eukaryotes is different from that in organisms without a nucleus (Prokaryote). There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.

The domain Eukaryota appears to be monophyletic, and so makes up one of the three domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things; even in a human body there are 10 times more microbes than human cells. However, due to their much larger size, their collective worldwide biomass is estimated at about equal to that of prokaryotes. Eukaryotes first developed approximately 1.6–2.1 billion years ago.

1200 Ma

Sexual reproduction evolves, leading to faster evolution.

900 Ma




The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. As the name suggests, choanoflagellates (collared flagellates) have a distinctive cell morphology characterized by an ovoid or spherical cell body 3–10 µm in diameter with a single apical flagellum surrounded by a collar of 30–40 microvilli. Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap bacteria and detritus against the collar of microvilli where these foodstuffs are engulfed. This feeding provides a critical link within the global carbon cycle, linking trophic levels. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in animals. As the closest living relatives of animals, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals.

Each choanoflagellate has a single flagellum, surrounded by a ring of actin-filled protrusions called microvilli, forming a cylindrical or conical collar (choanos in Greek). Movement of the flagellum draws water through the collar, and bacteria and detritus are captured by the microvilli and ingested. Water currents generated by the flagellum also push free-swimming cells along, as in animal sperm. In contrast, most other flagellates are pulled by their flagella.

In addition to the single apical flagellum surrounded by actin-filled microvilli that characterizes choanoflagellates, the internal organization of organelles in the cytoplasm is constant. A flagellar basal body sits at the base of the apical flagellum, and a second, non-flagellar basal body rests at a right angle to the flagellar base. The nucleus occupies an apical-to-central position in the cell, and food vacuoles are positioned in the basal region of the cytoplasm. Additionally, the cell body of many choanoflagellates is surrounded by a distinguishing extracellular matrix or periplast. These cell coverings vary greatly in structure and composition and are used by taxonomists for classification purposes. Many choanoflagellates build complex basket-shaped “houses” called lorica, from several silica strips cemented together. The functional significance of the periplast is unknown, but in sessile organisms, it is thought to aid in attachment to the substrate. In planktonic organisms, there is speculation that the periplast increases drag, thereby counteracting the force generated by the flagellum and increasing feeding efficiency. Choanoflagellates are either free-swimming in the water column or sessile, adhering to the substrate directly or through either the periplast or a thin pedicel. Although choanoflagellates are thought to be strictly free-living and heterotrophic, a number of choanoflagellate relatives such as members of Ichthyosporea or Mesomycetozoa follow a parasitic or pathogenic lifestyle. The life histories of choanoflagellates are poorly understood. Many species are thought to be solitary; however coloniality seems to have arisen independently several times within the group and colonial species retain a solitary stage.


600 Ma



Sponge-like creature

It is thought that the earliest multicellular animal was a sponge-like creature.

Sponges are among the simplest of animals, with partially differentiated tissues.

Sponges (Porifera) are the phylogenetically oldest animal phylum extant today.









580 Ma



Animal movement may have started with cnidarians. Almost all cnidarians possess nerves and muscles. Because they are the simplest animals to possess them, their direct ancestors were very probably the first animals to use nerves and muscles together. Cnidarians are also the first animals with an actual body of definite form and shape. They have radial symmetry. The first eyes evolved at this time.



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