The history of life on Earth

Earth is about 4.54 billion years old, and life has been present for most of that time. The history of life is written in rocks, in fossils and in the genomes of living things. This lesson covers how scientists date rocks and fossils, the major chapters of life's history from the first cells to the present, the mass extinctions that repeatedly reset the living world, and some of the remarkable fossil sites that let us read the story. Lesson 6 then shows how to reconstruct the branching relationships among the organisms involved.

Reading the rock record

Most fossils are found in sedimentary rocks, which form when layers of sand, mud or other particles settle and harden. The principle of superposition, set out by Nicolas Steno in the 1600s, states that in undisturbed layers the lower ones are older. This gives relative dates: we can say one fossil is older than another without knowing either age in years. Geologists matched layers across regions using distinctive index fossils, building the geological time scale in the 1800s long before absolute ages were known.

Fossilization is rare. Most organisms decay or are eaten; hard parts such as shells, bones and teeth fossilize far more often than soft tissues; and organisms living where sediment accumulates, such as shallow seas and river floodplains, are much better represented than those in mountains or forests. The fossil record is therefore incomplete and biased, but it is still enormous, and its overall order has been confirmed again and again. Fossils include not only petrified remains but also molds and casts, footprints and burrows (trace fossils), amber-preserved insects, frozen mammoths and molecular traces.

Radiometric dating

Absolute ages come from radiometric dating, which uses the steady decay of radioactive isotopes. Each radioactive isotope (the parent) decays into a daughter product at a fixed rate, described by its half-life: the time for half of the parent atoms in a sample to decay. Measuring the ratio of parent to daughter in a mineral tells how long ago the mineral formed.

Carbon-14 dating works differently from the others. Living things take in carbon-14 from the atmosphere while alive, keeping a roughly constant proportion; after death the carbon-14 decays without replacement. It is useful only for remains younger than about 50,000 years. Most fossils are far older, so they are dated indirectly: sedimentary rocks cannot usually be dated radiometrically, but volcanic ash layers above and below a fossil can, bracketing its age.

import math
# Radiometric ages from the fraction of parent isotope remaining
def age(fraction_left, half_life):
    return half_life * math.log2(1 / fraction_left)

print(f"Charcoal with 25% of its original C-14: {age(0.25, 5730):,.0f} years")
print(f"Bone with 10% of its original C-14:     {age(0.10, 5730):,.0f} years")
# K-Ar: each decayed K-40 atom here is assumed to leave one Ar-40 atom (simplified)
ar_per_k = 0.0035                     # ratio of Ar-40 to remaining K-40 in a volcanic ash
fraction_left = 1 / (1 + ar_per_k)
print(f"Ash with Ar/K = {ar_per_k}: {age(fraction_left, 1.25e9)/1e6:,.1f} million years")

Charcoal with 25% of its original C-14: 11,460 years
Bone with 10% of its original C-14:     19,035 years
Ash with Ar/K = 0.0035: 6.3 million years

Real potassium–argon dating must also account for the fact that only about 11 percent of potassium-40 decays to argon-40 (the rest becomes calcium-40), and modern laboratories use the more precise argon–argon method. But the principle is exactly as shown: a measured ratio and a known half-life give an age.

The geological time scale

Earth's history is divided into eons, eras, periods and epochs, with boundaries often marked by major changes in the fossil record. The Phanerozoic eon, meaning visible life, covers the last 539 million years, when animals with hard parts became abundant. Everything before it, nearly nine-tenths of Earth's history, is informally called the Precambrian.

Deep time is hard to grasp. One common way is to compress Earth's history into a single calendar year. The code below does that conversion.

from datetime import datetime, timedelta
# Compress Earth's 4,540-million-year history into one calendar year
EARTH = 4540e6
start = datetime(2026, 1, 1)
events = [("first life", 3700e6), ("oxygen builds up", 2400e6), ("first eukaryotes", 1800e6),
          ("Cambrian explosion", 539e6), ("plants on land", 470e6), ("dinosaurs extinct", 66e6),
          ("Homo sapiens", 0.3e6)]
for name, ago in events:
    t = start + timedelta(days=365 * (1 - ago / EARTH))
    print(f"{name:<20} {t:%B %d, %H:%M}")

first life           March 09, 12:47
oxygen builds up     June 22, 01:09
first eukaryotes     August 09, 06:52
Cambrian explosion   November 18, 15:59
plants on land       November 24, 05:07
dinosaurs extinct    December 26, 16:39
Homo sapiens         December 31, 23:25

On this calendar, life appears in early March, but animals with shells and skeletons do not appear until mid-November, the dinosaurs die out on December 26 and our own species shows up only in the last 35 minutes of December 31. For most of its history, life on Earth was entirely microbial.

The origin of life

How life began is still an open research question, but there has been real progress. In 1953 Stanley Miller and Harold Urey showed that amino acids form spontaneously when electric sparks pass through a mixture of gases thought to resemble the early atmosphere. Later work has shown that nucleotides, lipids that self-assemble into membrane-bound vesicles, and short RNA chains can all form under plausible early-Earth conditions.

A leading hypothesis is the RNA world: early life may have used RNA both to store information, as DNA does now, and to speed up chemical reactions, as protein enzymes do now. Ribozymes, RNA molecules with catalytic activity, still exist; the core of the ribosome that builds every protein is itself RNA. Candidate settings include warm shallow ponds that dry and refill and deep-sea hydrothermal vents, where mineral surfaces and chemical gradients could have driven early metabolism. What is clear is that by about 3.5 billion years ago, microbial life was well established.

Mass extinctions

Background extinction, the steady loss of species, goes on all the time; the average species lasts perhaps one to ten million years. Five times in the Phanerozoic, extinction rates spiked dramatically, removing large fractions of all species in geologically short intervals.

Evidence for the end-Cretaceous impact includes a worldwide thin layer of clay rich in iridium, an element rare in Earth's crust but common in asteroids, first reported by Luis and Walter Alvarez in 1980, and the 180-kilometer Chicxulub crater buried beneath the Yucatán Peninsula. Mass extinctions matter for evolution because they clear ecological space; the survivors then radiate. Mammals had lived alongside dinosaurs for over 100 million years as mostly small animals, but after the extinction they diversified rapidly into whales, bats, horses, primates and much more.

Many biologists argue that human activity is now driving a sixth mass extinction. Current extinction rates for well-studied groups such as mammals, birds and amphibians are estimated at tens to hundreds of times the background rate, driven by habitat loss, overexploitation, invasive species, pollution and climate change. Lesson 12 returns to biodiversity loss and conservation.

Windows into the past: exceptional fossil sites

Most fossil sites preserve only hard parts, but a few, called Lagerstätten, preserve soft tissues in remarkable detail. They give glimpses of whole ancient communities.

The United States holds some of the world’s most important fossil sites. The Morrison Formation of Colorado, Utah and Wyoming yielded famous Jurassic dinosaurs such as Stegosaurus, Allosaurus and giant sauropods; Dinosaur National Monument preserves a wall of bones in place. The Hell Creek Formation of Montana and the Dakotas records the last dinosaurs, including Tyrannosaurus and Triceratops, right up to the end-Cretaceous boundary. The Green River Formation of Wyoming preserves 50-million-year-old fish, birds and plants in fine detail, and the La Brea Tar Pits in Los Angeles hold tens of thousands of Ice Age mammals. The Paleontological Resources Preservation Act of 2009 requires permits to collect vertebrate fossils on federal land, while fossils on private land generally belong to the landowner. Canada’s Burgess Shale in British Columbia remains the classic window into Cambrian life.

Each of these sites tells part of the same story. Their fossils appear in the order evolutionary theory predicts: no rabbits in the Cambrian, no dinosaurs below the first reptiles, no flowering plants before the first seed plants. In over 150 years of collecting, no fossil has turned up seriously out of sequence.