Earth Research CardsFrom the Solar System to Complex Life

Real basis

• The first exoplanet: A giant planet was found around 51 Pegasi, circling its star in 4 days. The solar system was no longer unique. Nobel Prize 2019.

  Source: Mayor & Queloz, Nature 1995, "A Jupiter-mass companion to a solar-type star"

• Birth of the Moon: Computer simulations show a Mars-sized body hit Earth at a slant, and the debris came together to form the Moon.

  Source: Canup & Asphaug, Nature 2001, "Origin of the Moon in a giant impact near the end of the Earth's formation"

• The Nice model: A violent reshuffling of the giant planets' orbits flung many small bodies into the inner solar system, causing the Late Heavy Bombardment.

  Source: Gomes, Levison, Tsiganis & Morbidelli, Nature 2005, "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets"

• Jupiter's Grand Tack: Jupiter once migrated inward and was “pulled back” by Saturn, eating Mars's raw material on the way, which is why Mars is so small.

  Source: Walsh et al., Nature 2011, "A low mass for Mars from Jupiter's early gas-driven migration"

• The ocean 4.4 billion years ago: A zircon the size of a grain of sand suggests Earth may have had liquid water only about 150 million years after it formed.

  Source: Wilde, Valley, Peck & Graham, Nature 2001, "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago"

• The age of the Earth: Patterson used lead isotopes in meteorites to date Earth at about 4.55 billion years. To eliminate lead pollution, he later pushed for a ban on leaded gasoline.

  Source: Patterson, Geochimica et Cosmochimica Acta 1956, "Age of meteorites and the earth"

• The aluminum-26 puzzle: Decay products of aluminum-26 in meteorites show that a star had just died nearby when the solar system was born.

  Source: Lee, Papanastassiou & Wasserburg, The Astrophysical Journal Letters 1977, "Aluminum-26 in the early solar system: Fossil or fuel?"

• Seven temperate planets: Seven Earth-sized planets crowd around a small red dwarf, and several may have liquid water.

  Source: Gillon et al., Nature 2017, "Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1"

• Kepler: The Kepler telescope stared at one patch of sky and looked for new worlds by the faint dimming when planets transit, eventually finding thousands.

  Source: Borucki et al., Science 2010, "Kepler planet-detection mission: introduction and first results"

• The Miller–Urey experiment: A graduate student sealed water, methane, ammonia and hydrogen in a flask and kept sparking it, and got the amino acids that make up proteins.

  Source: Miller, Science 1953, "A production of amino acids under possible primitive earth conditions"

• Nucleotides without enzymes: Found a more “natural” chemical route to directly make RNA components under primitive conditions.

  Source: Powner, Gerland & Sutherland, Nature 2009, "Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions"

• RNA that cuts itself: Cech found that RNA can catalyze reactions like an enzyme. RNA is both information and tool. Nobel Prize 1989.

  Source: Kruger, Cech et al., Cell 1982, "Self-splicing RNA: autoexcision and autocyclization of the ribosomal RNA intervening sequence of Tetrahymena"

• Ribonuclease P: Altman independently found that the part of one enzyme that really does the catalysis is RNA, not protein.

  Source: Guerrier-Takada, Altman et al., Cell 1983, "The RNA moiety of ribonuclease P is the catalytic subunit of the enzyme"

• RNA world: Gilbert proposed that the earliest life may have been made entirely of RNA, with DNA and proteins added later.

  Source: Gilbert, Nature 1986, "Origin of life: The RNA world"

• Self-replicating RNA: Two RNA enzymes copy each other, and their numbers can keep growing exponentially, a step closer to “molecular life.”

  Source: Lincoln & Joyce, Science 2009, "Self-sustained replication of an RNA enzyme"

• Amino acids in space: Amino acids of non-terrestrial origin were found in the Murchison meteorite. The raw materials of life may be common in the universe.

  Source: Kvenvolden et al., Nature 1970, "Evidence for extraterrestrial amino-acids and hydrocarbons in the Murchison meteorite"

• The ribosome is a ribozyme: The core of the ribosome's protein synthesis is RNA, direct evidence left by the RNA world. Nobel Prize 2009.

  Source: Nissen, Hansen, Ban, Moore & Steitz, Science 2000, "The structural basis of ribosome activity in peptide bond synthesis"

• Deep-sea hot springs: Ecosystems that do not depend on sunlight exist around deep-sea hot springs, giving the origin of life a new candidate site.

  Source: Corliss et al., Science 1979, "Submarine thermal springs on the Galápagos Rift"

• A third kind of life: Comparing ribosomal RNA showed that “bacteria” are actually two very different kinds of life, and archaea became their own branch.

  Source: Woese & Fox, PNAS 1977, "Phylogenetic structure of the prokaryotic domain: The primary kingdoms"

• The three-domain system: The tree of life was redivided into three domains: Bacteria, Archaea and Eukaryotes.

  Source: Woese, Kandler & Wheelis, PNAS 1990, "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya"

• Endosymbiotic theory: Margulis proposed that mitochondria and chloroplasts were once independent bacteria that were swallowed and became part of the cell. It was rejected by over a dozen journals.

  Source: Sagan (Margulis), Journal of Theoretical Biology 1967, "On the origin of mitosing cells"

• The home of LUCA: Working back from modern genes, the common ancestor of all life may have lived near hot, oxygen-free, hydrogen-rich deep-sea vents.

  Source: Weiss et al., Nature Microbiology 2016, "The physiology and habitat of the last universal common ancestor"

• Snowball Earth: Rock layers in Namibia show that about 700 million years ago Earth may have been almost completely covered in ice.

  Source: Hoffman et al., Science 1998, "A Neoproterozoic snowball Earth"

• The rise of oxygen: Oxygen in the atmosphere did not arrive all at once, but rose through a long, repeated process.

  Source: Lyons, Reinhard & Planavsky, Nature 2014, "The rise of oxygen in Earth's early ocean and atmosphere"

• The double helix: A one-page paper proposed that DNA is a double helix, and base pairing hinted at how genetic information is copied.

  Source: Watson & Crick, Nature 1953, "Molecular structure of nucleic acids: A structure for deoxyribose nucleic acid"

• Asgard archaea: Lokiarchaeota found in deep-sea sediments carry many genes once thought to exist only in eukaryotes.

  Source: Spang et al., Nature 2015, "Complex archaea that bridge the gap between prokaryotes and eukaryotes"

• The earliest sexual reproduction: Fossils of an ancient red alga show sexual reproduction and multicellular life have at least a billion years of history.

  Source: Butterfield, Paleobiology 2000, "Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes"

• 3.7-billion-year-old stromatolites: Rocks in Greenland may preserve microbial structures 3.7 billion years old, although this interpretation is still disputed.

  Source: Nutman et al., Nature 2016, "Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures"

• Natural selection: Darwin and Wallace jointly published the theory of natural selection. The next year, On the Origin of Species came out.

  Source: Darwin & Wallace, Journal of the Proceedings of the Linnean Society 1858, "On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection"

• The dinosaur extinction puzzle: Iridium is anomalously enriched in the end-Cretaceous layer, and the Alvarezes, father and son, concluded that an asteroid had hit Earth.

  Source: Alvarez et al., Science 1980, "Extraterrestrial cause for the Cretaceous-Tertiary extinction"

• Finding the crater: Buried under Mexico's Yucatán Peninsula is a crater about 180 km across, exactly what that asteroid left behind.

  Source: Hildebrand et al., Geology 1991, "Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico"

• Tiktaalik: Scientists predicted which age of rock the transitional fossil should be in, and then really found it in the Canadian Arctic.

  Source: Daeschler, Shubin & Jenkins, Nature 2006, "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan"

• The earliest animals: Dickinsonia fossils preserve cholesterol-type molecules, proving this mysterious creature was an animal.

  Source: Bobrovskiy et al., Science 2018, "Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals"

• The earliest vertebrates: Myllokunmingia and Haikouichthys found in Chengjiang, Yunnan, pushed the history of vertebrates back to the early Cambrian.

  Source: Shu et al., Nature 1999, "Lower Cambrian vertebrates from south China"

• Feathered dinosaurs: Fossils from Liaoning show that feathers grew on dinosaurs before birds appeared.

  Source: Ji, Currie, Norell & Ji, Nature 1998, "Two feathered dinosaurs from northeastern China"

• The first flower: Archaefructus from Liaoning was considered one of the earliest flowering plants, and its age was later revised to the Early Cretaceous.

  Source: Sun et al., Science 1998, "In search of the first flower: a Jurassic angiosperm, Archaefructus, from northeast China"

• Punctuated equilibrium: Species may stay stable for a long time and then change rapidly in a short time, rather than evolving at a steady rate.

  Source: Eldredge & Gould, Models in Paleobiology 1972, "Punctuated equilibria: an alternative to phyletic gradualism"

• The worst mass extinction: Precise dating shows the end-Permian extinction may have lasted only about 60,000 years, matching Siberian volcanic activity.

  Source: Burgess, Bowring & Shen, PNAS 2014, "High-precision timeline for Earth's most severe extinction"

• Whales that walked on land: Fossils from Pakistan show that the ancestors of whales were hoofed animals that ran on land, related to hippos.

  Source: Thewissen et al., Nature 2001, "Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls"