Complex lifeanimals, plants and the mass extinctions

The era of complex life begins 640 million years ago, when cells started to divide labor. First came soft, frond-shaped organisms. Then the Cambrian explosion brought eyes, shells and predators all at once. After that came vertebrates, plants moving onto land and tetrapods, with two mass extinctions interrupting along the way. The era lasts until 6 million years ago and has thirteen key events.

Thirteen events

• 640 million years ago · Multicellular organisms: Cells begin to specialize.
• 570 million years ago · Ediacaran biota: Soft, frond-shaped organisms.
• 540 million years ago · Cambrian explosion: Eyes, shells, predators.
• 530 million years ago · Vertebrates: The notochord becomes a spine.
• 470 million years ago · Plants move onto land: Moss climbs onto rock.
• 370 million years ago · Tetrapods: Fins become limbs.
• 310 million years ago · Amniotic egg: Eggs can leave the water.
• 250 million years ago · Permian mass extinction: 96% of marine species disappear.
• 230 million years ago · Age of dinosaurs: Reptiles rule the land.
• 130 million years ago · Flowering plants: Flowers and pollinating insects evolve together.
• 66 million years ago · Cretaceous extinction: An asteroid hits Chicxulub.
• 60 million years ago · Mammal radiation: Mammals take over the vacated niches.
• 55 million years ago · Primates: Tree-living, stereoscopic vision, grasping hands.

How evolution happens

The theory of natural selection was put forward by two people at the same time, and On the Origin of Species was published the following year (Darwin and Wallace, Journal of the Proceedings of the Linnean Society, 1858). More than a century later came an addition: species may stay stable for a long time and then change rapidly over a short period, rather than evolving at a steady pace. This is punctuated equilibrium (Eldredge and Gould, Models in Paleobiology, 1972).

The earliest animals and vertebrates

Cholesterol-like molecules preserved in Dickinsonia fossils prove that this mysterious organism was an animal, one of the earliest known (Bobrovskiy et al., Science, 2018). Myllokunmingia and Haikouichthys, found at Chengjiang in Yunnan, push the history of vertebrates back to the early Cambrian (Shu et al., Nature, 1999).

From water to land

Scientists predicted which age of rock should hold a transitional fossil, then really found Tiktaalik in the Canadian Arctic (Daeschler, Shubin and Jenkins, Nature, 2006). The story also runs the other way: fossils unearthed in Pakistan show that the ancestors of whales were land-running hoofed mammals related to hippos (Thewissen et al., Nature, 2001).

Two mass extinctions

The end-Permian mass extinction was the most severe on Earth. Precise dating shows it may have lasted only about 60,000 years, which fits the volcanic activity in Siberia (Burgess, Bowring and Shen, PNAS, 2014).

The end-Cretaceous extinction has a "detective line". An anomalous concentration of the element iridium in the rock layers led Luis and Walter Alvarez to infer that an asteroid had hit Earth (Alvarez et al., Science, 1980). More than ten years later, a crater about 180 km across was found buried beneath the Yucatán Peninsula in Mexico, exactly what that asteroid would have left (Hildebrand et al., Geology, 1991).

Feathers and flowers

Fossils from Liaoning show that feathers grew on dinosaurs before birds appeared (Ji, Currie, Norell and Ji, Nature, 1998). Archaefructus, also from Liaoning, is considered one of the earliest flowering plants, and its age was later revised to the Early Cretaceous (Sun et al., Science, 1998).

Want to read the originals? Search these titles

• Darwin and Wallace (1858): On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection
• Eldredge and Gould (1972): Punctuated equilibria: an alternative to phyletic gradualism
• Bobrovskiy et al. (2018): Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals
• Shu et al. (1999): Lower Cambrian vertebrates from south China
• Daeschler et al. (2006): A Devonian tetrapod-like fish and the evolution of the tetrapod body plan
• Thewissen et al. (2001): Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls
• Burgess et al. (2014): High-precision timeline for Earth's most severe extinction
• Alvarez et al. (1980): Extraterrestrial cause for the Cretaceous-Tertiary extinction
• Hildebrand et al. (1991): Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico
• Ji et al. (1998): Two feathered dinosaurs from northeastern China
• Sun et al. (1998): In search of the first flower: a Jurassic angiosperm, Archaefructus, from northeast China

How the game shows it

In the game, the Complex life era has the resource biomass, and its generators are Multicellular colonies, Ocean plankton, Land plants and Food web. They cover the main line from cell specialization to the formation of ecosystems. The two extinctions above are major interruptions along that line.