History of life

• 1. Introduction

  ›

  The history of life on Earth begins with a planet that formed about 4.54 billion years ago, give or take 50 million. Scientists abbreviate that span as Ga, for gigaannum, and they suspect life appeared before 3.7 Ga. Every species alive today, from a microbe to a whale, traces back to a common ancestor through evolution. The proof is in the resemblance. All known organisms share the same basic biochemical machinery. That shared inheritance raises hard questions. How did life cross the line from non-living chemistry to a thing that copies itself? Why did oxygen, a waste product, end up reshaping the whole planet? What made complex cells possible, and why does almost everything reproduce sexually when an asexual rival could out-breed it in fifty generations? Consider one striking estimate. Earth may hold a trillion species, yet only 1.75 to 1.8 million have been named. The currently living species represent less than one percent of all species that have ever lived. The rest are gone, and much of what follows is the story of who replaced whom.

• 2. Whispers In Ancient Rock

  ›

  Biogenic carbon signatures and stromatolite fossils sit inside 3.7 billion-year-old metasedimentary rocks from western Greenland, the earliest clear evidence of life. Older claims push further back and grow shakier. In 2015, possible remains of biotic life turned up in 4.1 billion-year-old rocks in Western Australia. Fossilized microorganisms in hydrothermal vent precipitates from the Nuvvuagittuq Belt in Quebec, Canada, may have lived as early as 4.28 billion years ago, though the evidence is disputed as inconclusive. The oceans had formed by 4.4 billion years ago, so life, if real, arrived soon after there was water to hold it. The earliest organisms make detective work brutal. They were minute and relatively featureless, and their fossils look like small rods nearly impossible to separate from structures abiotic physics can produce. The oldest undisputed evidence, read as fossilized bacteria, dates to 3 Ga. Older finds at roughly 3.5 Ga and geochemical hints at 3.8 Ga drew close scrutiny, and researchers found non-biological processes that could mimic every reported signature of life. That does not prove the structures were lifeless. It means they cannot stand as clear proof. Stromatolites add their own quarrel. Critics argue that alleged examples from before 3 Ga could be non-biological, though in 2006 a fresh find emerged from Australia in rocks dated to 3.5 Ga.

• 3. The Recipes For A Beginning

  ›

  Carbon and water are the foundation of life on Earth, and no substitute comes close. Carbon builds stable frameworks for complex chemicals and pulls easily from carbon dioxide. Silicon sits directly below carbon on the periodic table, yet it forms few complex stable molecules, and silicon dioxide is a hard abrasive solid rather than a convenient gas. Water adds two quiet gifts. Ice floats, so aquatic organisms survive beneath it in winter, and water's electrically charged ends let it form a wider range of compounds than rivals like ammonia. Researchers chasing the leap from chemistry to biology focus on three starting points. There is self-replication, the ability to make near-copies of oneself. There is metabolism, the ability to feed and repair. And there are external cell membranes that admit food, expel waste, and bar unwanted substances. From these threads come competing origin stories. The replication-first view imagines an RNA world, because some RNA molecules can catalyze both their own replication and the building of proteins. Such an RNA world would have had individuals but no species, with mutations and horizontal gene transfer making offspring genetically unlike their parents. Evidence suggests the first RNA molecules formed prior to 4.17 Ga. Rival accounts put membranes first, with lipid bubbles called liposomes that reproduce themselves, or hand the early work to clays. Montmorillonite clay grows by self-replicating its crystal pattern and can catalyze the formation of RNA molecules. A metabolism-first camp points to iron-sulfur and nickel-sulfide catalysts that, in experiments from 1997, began building proteins from carbon monoxide and hydrogen sulfide near hydrothermal vents.

• 4. Cradles And Distant Origins

  ›

  Wet-dry cycles at geothermal springs solve a stubborn problem, hydrolysis, and they push biopolymers to polymerize and pack into vesicles. Analysis of hydrothermal veins at a 3.5 Gya geothermal spring setting found the elements an origin needs, including potassium, boron, hydrogen, sulfur, phosphorus, zinc, nitrogen, and oxygen. Mulkidjanian and colleagues note that such freshwater settings carry ionic concentrations identical to the cytoplasm of modern cells. Deep sea hydrothermal vents offer a competing cradle. In simulated white-smoker conditions, scientists oligomerized RNA four units long, and in alkaline vent pores RNA molecules of 22 bases can be polymerized. A genomic analysis found 355 genes likely tracing to LUCA, the last universal common ancestor, among 6.1 million sequenced prokaryotic genes, and reconstructed LUCA as a thermophilic anaerobe with a Wood-Ljungdahl pathway. Carbonate-rich lakes answer a different shortage, phosphate, which natural environments deplete through microbial uptake and binding to calcium. In carbonate water, calcium locks into calcium carbonate instead of apatite, freeing phosphate to rise. Models put early lake phosphate at roughly 100 times today's levels. Some accounts skip Earth entirely. The panspermia hypothesis, the idea that life was seeded from elsewhere, dates back at least to the Greek philosopher Anaximander in the sixth century BCE, and was later proposed by Svante Arrhenius, by Fred Hoyle and Chandra Wickramasinghe, and by Francis Crick and Leslie Orgel. In low Earth orbit experiments such as EXOSTACK, some microorganism spores survived outer space radiation for at least 5.7 years.

• 5. Mats That Changed The Air

  ›

  Microbial mats are multi-layered, multi-species colonies only a few millimeters thick, yet each holds a range of chemical environments. The by-products of one group of microorganisms feed the next, so each mat runs its own small food chain. These mats of coexisting bacteria and archaea dominated the early Archean, and many major steps in early evolution likely happened inside them. Stromatolites grew from this world as stubby pillars, built when microorganisms migrated upward to escape sediment settling on them. Cyanobacteria invented photosynthesis around 3.5 Ga, and their waste product would remake the planet. Oxygenic photosynthesis by mat bacteria raised biological productivity by a factor between 100 and 1,000, drawing hydrogen from water rather than scarce geological reducing agents. From that point on, life produced more of its own resources than geochemistry did. Free oxygen first saturated every available reductant on Earth's surface, then spilled into the atmosphere, driving the Great Oxygenation Event around 2.4 Ga. Oxygen is toxic to organisms not adapted to it, yet it sharply boosts the efficiency of those that are. Inside the mats, the boundary between oxygen-rich and oxygen-free layers rose at night when photosynthesis stopped and sank again by day. That daily shift pressured organisms in the middle zone to tolerate oxygen and then to use it, possibly through endosymbiosis, where one organism lives inside another to mutual benefit.

• 6. Complex Cells And The Endosymbiont Bargain

  ›

  Mitochondria fuel the production of ATP, the internal energy supply of all known cells, and most modern eukaryotes cannot live without the oxygen those mitochondria burn. In the 1970s a vigorous debate settled on an answer for where complex cells came from. Eukaryotes arose through a sequence of endosymbiosis between prokaryotes. A predatory microorganism invaded a large prokaryote, probably an archaean, and instead of killing its prey it took up residence and became mitochondria. The earliest evidence of eukaryotes dates from 1.85 Ga, drawn from fossils of the alga Grypania in rocks of that age. Their diversification sped up once aerobic respiration by mitochondria delivered a richer supply of biological energy. Plastids tell the second chapter of this bargain. Around 1.5 Ga, a eukaryote tried to swallow a photosynthesizing cyanobacterium, the victim survived inside, and the pairing became the ancestor of plants. From that single event grew three lineages, chloroplasts in green algae and plants, rhodoplasts in red algae, and cyanelles in glaucophytes. Around 1.6 Ga, eukaryotes that gained photosynthesis gave rise to algae that eventually overtook cyanobacteria as the dominant primary producers. Dating remains contested. Steranes in Australian shales were once read as eukaryotes at 2.7 Ga, but a 2008 analysis concluded the chemicals infiltrated the rocks less than 2.2 Ga and prove nothing. The earliest known fossils of fungi date from 1.43 Ga.

• 7. The Sexual Puzzle And Bodies Of Many Cells

  ›

  Meiosis and fertilization define sexual reproduction in eukaryotes, giving offspring 50 percent of their genes from each parent through genetic recombination. The puzzle is why it persists, since an asexual population can out-breed and displace an otherwise equal sexual one in as little as 50 generations. Males, after all, do not directly add to the next generation's numbers. Several hypotheses circle the problem without closing it. The Red Queen hypothesis argues that sex defends against parasites by presenting a moving target, yet Kathryn A. Hanley and colleagues found mites significantly more prevalent in sexual geckos than in asexual ones sharing the same habitat. Alexey Kondrashov's deterministic mutation hypothesis assumes harmful mutations combine to do more damage than their sum, so recombination can purge them, but many species average less than one harmful mutation per individual. John A. Birdsell and Christopher Wills reviewed the competing models, and one alternative holds that sex arose to repair DNA damage, with genetic variety as a byproduct. Sexual reproduction also opened the door to complex bodies. Multicellularity evolved independently in sponges and other animals, fungi, plants, brown algae, cyanobacteria, slime molds, and myxobacteria. Sexual reproduction eliminates rogue cells that would otherwise reduce an asexual organism to undifferentiated mush, so it appears to be a prerequisite for complex multicellularity. The red alga Bangiomorpha, dated at 1.2 Ga, is the earliest known organism with truly differentiated, specialized cells, and also the oldest known sexually reproducing organism.

• 8. Conquering The Land And The Long Handoff

  ›

  Bilateria, animals with mirror-image left and right sides, appeared by 555 Ma, and most modern phyla originated during the Cambrian explosion. In November 2019 researchers reported Caveasphaera from 609-million-year-old rocks, a multicellular organism not easily called animal or non-animal, hinting animal evolution may have begun about 750 million years ago. The Ediacara biota flourished for the last 40 million years before the Cambrian, the first animals more than a very few centimeters long. The Chengjiang fossils Haikouichthys and Myllokunmingia appear to be true vertebrates, and Haikouichthys had distinct vertebrae. Moving onto land demanded near-total redesign. Organisms had to avoid drying out, withstand gravity, rework respiration, and free reproduction from water. Modern land ecosystems only appeared in the Late Devonian. Trees such as Archaeopteris bound the soil so firmly that rivers shifted from braided to meandering, triggering the Late Devonian wood crisis. By drawing down carbon dioxide they reduced the greenhouse effect and helped cause a Carboniferous ice age, while their deepening roots washed nutrients into water and fed anoxic algal blooms. Acanthostega, about 1 meter long, was a Late Devonian animal with legs, lungs, and gills, yet too weak to survive on land, a wholly aquatic predator of shallow water. The Permian-Triassic extinction event wiped out almost all land vertebrates, and recovery took an estimated 30 million years. From it rose the archosaurs, and one group, the dinosaurs, ruled the Jurassic and Cretaceous. After the Cretaceous-Paleogene extinction killed the non-avian dinosaurs, mammals increased rapidly in size and diversity, with bats taking to the air within 13 million years and cetaceans to the sea within 15 million years. Modern humans evolved from upright-walking apes traced back to Sahelanthropus, and the first known stone tools were made apparently by Australopithecus garhi. Mass extinctions rarely promote a superior group. They simply clear the old dominant one and make room, and the fossil record suggests the gaps between such catastrophes have been growing longer.