Early lifewhat the first living things looked like

All life on Earth comes from one last universal common ancestor, LUCA, which lived 3.7 billion years ago. The tree of life then split for the first time into bacteria and archaea. 3 billion years ago cyanobacteria began releasing oxygen. 2.4 billion years ago the atmosphere contained oxygen and many anaerobic organisms died out. 2 billion years ago one cell swallowed another without digesting it, and mitochondria came from this.

The time range of this era is "3.8 billion – 700 million years ago", and its events run from the earliest common ancestor to a global ice age.

Nine events

• 3.7 billion years ago · LUCA: The last common ancestor of all life.
• 3.5 billion years ago · Bacteria and archaea: The tree of life splits for the first time.
• 3.5 billion years ago · Stromatolites: Microbial mats leave fossils.
• 3 billion years ago · Oxygenic photosynthesis: Cyanobacteria begin to release oxygen.
• 2.4 billion years ago · Great Oxidation Event: The atmosphere contains oxygen and many anaerobic organisms die out.
• 2 billion years ago · Mitochondrial endosymbiosis: One cell swallows another without digesting it.
• 1.8 billion years ago · Eukaryotic cells: Cells get a nucleus.
• 1.2 billion years ago · Sexual reproduction: Gene recombination speeds up evolution.
• 700 million years ago · Snowball Earth: Ice sheets cover almost the whole globe.

The common ancestor and the tree of life

Working backward from modern genes, the common ancestor of all life may have lived near deep-sea hydrothermal vents that were hot, oxygen-poor and rich in hydrogen (Weiss et al., Nature Microbiology, 2016). This study is about the physiology and habitat of the last universal common ancestor. Rocks in Greenland may preserve microbial structures 3.7 billion years old, but this reading is still disputed, so the uncertainty should be kept when citing it (Nutman et al., Nature, 2016). The paper's title stresses how rapidly life appeared.

A change in classification came from molecular comparison: comparing ribosomal RNA showed that what we called "bacteria" are really two quite different kinds of life, and archaea became a separate branch (Woese and Fox, PNAS, 1977). The tree of life was then redivided into three domains: Bacteria, Archaea and Eukarya (Woese, Kandler and Wheelis, PNAS, 1990). Lokiarchaeota, found in deep-sea sediments, carry many genes once thought to exist only in eukaryotes (Spang et al., Nature, 2015).

Oxygen and eukaryotic cells

Oxygen in the atmosphere did not arrive all at once. It rose over a long period, with setbacks, so the "Great Oxidation Event" should not be read as a change overnight (Lyons, Reinhard and Planavsky, Nature, 2014). Mitochondria and chloroplasts were once independent bacteria that became part of the cell after being swallowed. This theory was rejected by more than a dozen journals at the time (Lynn Margulis, published as Sagan, Journal of Theoretical Biology, 1967).

Sexual reproduction and Snowball Earth

Fossils of an ancient red alga show that sexual reproduction and multicellular life have a history of at least 1 billion years (Butterfield, Paleobiology, 2000). Rock layers in Namibia show that about 700 million years ago, Earth may have been almost completely covered in ice (Hoffman et al., Science, 1998).

The genetic material: DNA's double helix

A one-page paper proposed that the genetic material DNA is a double helix, and that base pairing hints at how genetic information is copied (Watson and Crick, Nature, 1953).

Want to read the originals? Search these titles

• Weiss et al. (2016): The physiology and habitat of the last universal common ancestor
• Nutman et al. (2016): Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures
• Woese and Fox (1977): Phylogenetic structure of the prokaryotic domain: The primary kingdoms
• Woese et al. (1990): Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya
• Spang et al. (2015): Complex archaea that bridge the gap between prokaryotes and eukaryotes
• Lyons et al. (2014): The rise of oxygen in Earth's early ocean and atmosphere
• Sagan (1967): On the origin of mitosing cells
• Butterfield (2000): Bangiomorpha pubescens n. gen., n. sp., a paper on sexual reproduction, multicellularity and the eukaryote radiation
• Hoffman et al. (1998): A Neoproterozoic snowball Earth
• Watson and Crick (1953): Molecular structure of nucleic acids: A structure for deoxyribose nucleic acid

How the game shows it

In the game, the Early life era has the resource cells, and its generators are Prokaryote fission, Chemosynthesis, Cyanobacterial photosynthesis and Horizontal gene transfer. They run from the simplest cell division, to living on chemical energy, to using light to make oxygen, and finally to genes moving between different organisms.