9  Coevolution of Earth and Life

How did life change Earth, and how did Earth change life?

HS-ESS2-7 7–8 Days 🧠 Quiz & Evaluate ↓

🦠🌍 Did Life Create the Earth We Know?

10 🔥 Engage — The Oxygen Mystery

10.1 Earth’s Atmosphere Has Changed Dramatically

Today, Earth’s atmosphere is about 21% oxygen. But for the first 2 billion years of Earth’s history, there was almost no free oxygen at all. Where did all the oxygen come from?

Here’s the shocking answer: Life made it.

Tiny cyanobacteria (blue-green algae) invented photosynthesis and literally poisoned the planet with oxygen — killing off most of the anaerobic life that existed before them. This was the greatest pollution event in Earth’s history.

Plot = require("@observablehq/plot")
d3 = require("d3@7")

oxygenData = [
  {time: 4.5, o2: 0, era: "Hadean"},
  {time: 4.0, o2: 0, era: "Early Archean"},
  {time: 3.5, o2: 0.001, era: "First photosynthesis"},
  {time: 2.7, o2: 0.01, era: "Cyanobacteria spread"},
  {time: 2.4, o2: 2, era: "Great Oxidation Event"},
  {time: 2.0, o2: 3, era: "O₂ rises slowly"},
  {time: 1.5, o2: 4, era: "Boring Billion"},
  {time: 1.0, o2: 5, era: "Still low"},
  {time: 0.8, o2: 8, era: "Neoproterozoic rise"},
  {time: 0.6, o2: 15, era: "Pre-Cambrian surge"},
  {time: 0.54, o2: 15, era: "Cambrian Explosion"},
  {time: 0.4, o2: 21, era: "Land plants appear"},
  {time: 0.3, o2: 35, era: "Carboniferous peak"},
  {time: 0.25, o2: 15, era: "Permian extinction"},
  {time: 0.2, o2: 15, era: "Triassic"},
  {time: 0.1, o2: 20, era: "Cretaceous"},
  {time: 0.0, o2: 21, era: "Today"}
]

Plot.plot({
  title: "Atmospheric Oxygen Over Earth's History",
  subtitle: "Life transformed Earth's atmosphere — and mass extinctions crashed it",
  width: 700,
  height: 380,
  marginBottom: 50,
  x: {label: "Billions of years ago", reverse: true, grid: true},
  y: {label: "Oxygen (% of atmosphere)", grid: true, domain: [0, 40]},
  marks: [
    Plot.areaY(oxygenData, {x: "time", y: "o2", fill: "#74b9ff", fillOpacity: 0.3, curve: "natural",
        tip: true}),
    Plot.line(oxygenData, {x: "time", y: "o2", stroke: "#0984e3", strokeWidth: 3, curve: "natural",
        tip: true}),
    Plot.dot(oxygenData, {x: "time", y: "o2", fill: "#0984e3", r: 4,
        tip: true}),
    Plot.ruleY([21], {stroke: "#2ecc71", strokeWidth: 2, strokeDasharray: "8,4"}),
    Plot.text([{x: 0.15, y: 23}], {x: "x", y: "y", text: d => "Today's level (21%)", fill: "#2ecc71", fontSize: 11, fontWeight: "bold"}),
    // Great Oxidation Event marker
    Plot.ruleX([2.4], {stroke: "#e74c3c", strokeWidth: 2, strokeDasharray: "5,3"}),
    Plot.text([{x: 2.4, y: 37}], {x: "x", y: "y", text: d => "Great Oxidation\nEvent", fill: "#e74c3c", fontSize: 10, fontWeight: "bold", textAnchor: "middle"})
  ]
})

10.1.1 📝 Engage Questions

1. What happened to atmospheric oxygen at 2.4 billion years ago? What caused this change?
2. Look at the Carboniferous peak (~300 Mya) — why might oxygen have been so high then?
3. Why does oxygen drop at each mass extinction?

11 🔍 Explore 1 — The Fossil Record

11.1 4 Billion Years of Evolution

Life on Earth has been evolving for at least 3.8 billion years. The fossil record shows us when major groups of organisms first appeared — and when they went extinct.

evolutionData = [
  {event: "First life (prokaryotes)", time: 3800, complexity: 1, type: "Origin", color: "#2ecc71"},
  {event: "Photosynthesis evolves", time: 3500, complexity: 1.5, type: "Innovation", color: "#00cec9"},
  {event: "Great Oxidation Event", time: 2400, complexity: 2, type: "Crisis", color: "#e74c3c"},
  {event: "First eukaryotes", time: 2000, complexity: 3, type: "Origin", color: "#2ecc71"},
  {event: "First multicellular life", time: 1500, complexity: 4, type: "Origin", color: "#2ecc71"},
  {event: "Ediacaran fauna", time: 635, complexity: 5, type: "Origin", color: "#2ecc71"},
  {event: "Cambrian Explosion", time: 540, complexity: 8, type: "Innovation", color: "#00cec9"},
  {event: "First land plants", time: 470, complexity: 9, type: "Origin", color: "#2ecc71"},
  {event: "First insects", time: 400, complexity: 9, type: "Origin", color: "#2ecc71"},
  {event: "First forests", time: 360, complexity: 10, type: "Innovation", color: "#00cec9"},
  {event: "Permian extinction (96% lost)", time: 252, complexity: 4, type: "Extinction", color: "#d63031"},
  {event: "First dinosaurs", time: 230, complexity: 7, type: "Origin", color: "#2ecc71"},
  {event: "First mammals", time: 200, complexity: 8, type: "Origin", color: "#2ecc71"},
  {event: "First flowering plants", time: 130, complexity: 9, type: "Innovation", color: "#00cec9"},
  {event: "K-Pg extinction (dinosaurs)", time: 66, complexity: 5, type: "Extinction", color: "#d63031"},
  {event: "Mammals diversify", time: 55, complexity: 9, type: "Origin", color: "#2ecc71"},
  {event: "First hominids", time: 6, complexity: 10, type: "Origin", color: "#2ecc71"},
  {event: "Modern humans", time: 0.3, complexity: 10, type: "Origin", color: "#f39c12"}
]

Plot.plot({
  title: "Major Events in the History of Life",
  subtitle: "Millions of years ago — notice how extinctions are followed by diversification",
  width: 700,
  height: 450,
  marginLeft: 210,
  x: {label: "Millions of years ago", reverse: true, grid: true},
  y: {label: null},
  color: {legend: true, domain: ["Origin", "Innovation", "Crisis", "Extinction"], range: ["#2ecc71", "#00cec9", "#e74c3c", "#d63031"]},
  marks: [
    Plot.dot(evolutionData, {
      x: "time",
      y: "event",
      fill: "type",
      r: 7,
      stroke: "white",
      strokeWidth: 1
    ,
        tip: true})
  ]
})

🧠 The Cambrian Explosion (540 Mya) produced nearly every major animal body plan we see today — in just 20 million years. That’s fast in geological time! But it couldn’t have happened without oxygen levels rising above ~15% first.

12 💡 Explain — Dynamic Feedbacks

12.1 Life and Earth Changed Each Other

Earth and life didn’t evolve independently. They are locked in feedback loops where changes in one drive changes in the other.

12.2 The Key Feedback Loops

{
  const svg = d3.create("svg")
    .attr("width", 700)
    .attr("height", 550)
    .attr("viewBox", "0 0 700 550");

  svg.append("text").attr("x", 350).attr("y", 25).attr("text-anchor", "middle")
    .attr("font-size", 18).attr("font-weight", "bold").text("Earth-Life Feedback Loops");

  // Feedback loop 1: Photosynthesis-Oxygen
  const loop1Y = 80;
  svg.append("rect").attr("x", 20).attr("y", loop1Y).attr("width", 660).attr("height", 120)
    .attr("rx", 15).attr("fill", "#00b894").attr("opacity", 0.15);
  svg.append("text").attr("x", 40).attr("y", loop1Y + 25).attr("font-size", 14).attr("font-weight", "bold").attr("fill", "#00b894")
    .text("Loop 1: Photosynthesis ↔ Oxygen ↔ Complexity");
  
  const loop1Items = [
    {text: "🦠 Cyanobacteria", x: 110, y: loop1Y + 70},
    {text: "→ produce O₂ →", x: 260, y: loop1Y + 70},
    {text: "🌬️ O₂ rises", x: 390, y: loop1Y + 70},
    {text: "→ enables →", x: 490, y: loop1Y + 70},
    {text: "🐟 Complex life", x: 610, y: loop1Y + 70}
  ];
  loop1Items.forEach(item => {
    svg.append("text").attr("x", item.x).attr("y", item.y).attr("text-anchor", "middle")
      .attr("font-size", 13).attr("font-weight", "bold").text(item.text);
  });
  svg.append("text").attr("x", 350).attr("y", loop1Y + 100).attr("text-anchor", "middle")
    .attr("font-size", 11).attr("fill", "#636e72").text("More complex life → more photosynthesis → even more oxygen (positive feedback)");

  // Feedback loop 2: Weathering-CO2
  const loop2Y = 220;
  svg.append("rect").attr("x", 20).attr("y", loop2Y).attr("width", 660).attr("height", 120)
    .attr("rx", 15).attr("fill", "#6c5ce7").attr("opacity", 0.15);
  svg.append("text").attr("x", 40).attr("y", loop2Y + 25).attr("font-size", 14).attr("font-weight", "bold").attr("fill", "#6c5ce7")
    .text("Loop 2: Weathering ↔ CO₂ ↔ Climate (Thermostat)");
  
  svg.append("text").attr("x", 350).attr("y", loop2Y + 60).attr("text-anchor", "middle")
    .attr("font-size", 13).text("🌡️ Warmer → faster weathering → more CO₂ removed → cooler");
  svg.append("text").attr("x", 350).attr("y", loop2Y + 85).attr("text-anchor", "middle")
    .attr("font-size", 13).text("❄️ Cooler → slower weathering → CO₂ builds up → warmer");
  svg.append("text").attr("x", 350).attr("y", loop2Y + 110).attr("text-anchor", "middle")
    .attr("font-size", 11).attr("fill", "#636e72").text("This negative feedback has kept Earth's temperature relatively stable for billions of years");

  // Feedback loop 3: Extinction-Recovery
  const loop3Y = 360;
  svg.append("rect").attr("x", 20).attr("y", loop3Y).attr("width", 660).attr("height", 120)
    .attr("rx", 15).attr("fill", "#e74c3c").attr("opacity", 0.15);
  svg.append("text").attr("x", 40).attr("y", loop3Y + 25).attr("font-size", 14).attr("font-weight", "bold").attr("fill", "#e74c3c")
    .text("Loop 3: Extinction → Open Niches → Diversification");

  const loop3Items = [
    {text: "💥 Mass Extinction", x: 130, y: loop3Y + 70},
    {text: "→ empty niches →", x: 290, y: loop3Y + 70},
    {text: "🌱 Survivors diversify", x: 455, y: loop3Y + 70},
    {text: "→ new species", x: 600, y: loop3Y + 70}
  ];
  loop3Items.forEach(item => {
    svg.append("text").attr("x", item.x).attr("y", item.y).attr("text-anchor", "middle")
      .attr("font-size", 13).attr("font-weight", "bold").text(item.text);
  });
  svg.append("text").attr("x", 350).attr("y", loop3Y + 100).attr("text-anchor", "middle")
    .attr("font-size", 11).attr("fill", "#636e72").text("Without the K-Pg extinction, mammals (and humans) might never have dominated Earth");

  // Key takeaway
  svg.append("rect").attr("x", 100).attr("y", 500).attr("width", 500).attr("height", 40)
    .attr("rx", 20).attr("fill", "#00cec9");
  svg.append("text").attr("x", 350).attr("y", 525).attr("text-anchor", "middle")
    .attr("font-size", 14).attr("font-weight", "bold").attr("fill", "white")
    .text("Life and Earth are a coupled system — each drives the other");

  return svg.node();
}

12.2.1 💡 Coevolution = Earth Changed Life AND Life Changed Earth

This isn’t a one-way street:

• Life → Earth: Photosynthesis created an oxygen atmosphere, organisms built limestone and coal deposits, roots accelerated weathering
• Earth → Life: Plate tectonics created new habitats and barriers, volcanic eruptions caused mass extinctions, climate shifts drove adaptation
• Together: The carbon cycle, oxygen cycle, and nitrogen cycle are all driven by the interaction between geological and biological processes

13 🔍 Explore 2 — The Five Mass Extinctions

13.1 Destruction Creates Opportunity

Earth has experienced five major mass extinctions. Each one wiped out 60–96% of all species. But each extinction was followed by an explosion of new life forms filling the empty ecological niches.

extinctions = [
  {name: "End-Ordovician", time: 445, loss: 86, cause: "Ice age, sea level drop", survivors: "Fish, early land plants", color: "#74b9ff"},
  {name: "Late Devonian", time: 372, loss: 75, cause: "Ocean oxygen loss", survivors: "Early tetrapods, insects", color: "#0984e3"},
  {name: "End-Permian\n(The Great Dying)", time: 252, loss: 96, cause: "Massive volcanism, warming", survivors: "A few reptile lineages", color: "#d63031"},
  {name: "End-Triassic", time: 201, loss: 80, cause: "Volcanism, climate change", survivors: "Dinosaurs take over", color: "#e17055"},
  {name: "End-Cretaceous\n(K-Pg)", time: 66, loss: 76, cause: "Asteroid impact", survivors: "Mammals, birds", color: "#fdcb6e"}
]

Plot.plot({
  title: "The Big Five Mass Extinctions",
  subtitle: "Percentage of species lost — the End-Permian nearly ended all life",
  width: 700,
  height: 300,
  marginLeft: 160,
  x: {label: "Species lost (%)", grid: true, domain: [0, 100]},
  y: {label: null},
  marks: [
    Plot.barX(extinctions, {
      x: "loss",
      y: "name",
      fill: "color",
      rx: 8,
      sort: {y: "-x"}
    ,
        tip: true}),
    Plot.text(extinctions, {
      x: "loss",
      y: "name",
      text: d => d.loss + "%",
      dx: 15,
      fontSize: 14,
      fontWeight: "bold"
    })
  ]
})

viewof selectedExtinction = Inputs.select(
  extinctions.map(e => e.name.replace("\n", " ")),
  {label: "Explore an extinction event:"}
)

{
  const ext = extinctions.find(e => e.name.replace("\n", " ") === selectedExtinction);
  const div = d3.create("div");
  div.append("h3").style("color", "#00cec9").style("font-family", "Space Grotesk")
    .text(`${ext.name.replace("\n", " ")} — ${ext.time} Mya`);
  div.append("div").style("display", "flex").style("gap", "20px").style("flex-wrap", "wrap");
  
  const card1 = div.append("div").style("flex", "1").style("min-width", "200px")
    .style("background", "#ffe0e0").style("padding", "15px").style("border-radius", "10px");
  card1.append("h4").style("margin-top", "0").text("💀 What Happened");
  card1.append("p").text(`${ext.loss}% of species went extinct`);
  card1.append("p").style("font-weight", "bold").text(`Cause: ${ext.cause}`);
  
  const card2 = div.append("div").style("flex", "1").style("min-width", "200px")
    .style("background", "#e0ffe0").style("padding", "15px").style("border-radius", "10px");
  card2.append("h4").style("margin-top", "0").text("🌱 What Came Next");
  card2.append("p").text(`Survivors: ${ext.survivors}`);
  card2.append("p").style("font-weight", "bold")
    .text("These survivors diversified to fill empty niches, producing new groups of organisms.");
  
  return div.node();
}

🧠 Without the asteroid that killed the dinosaurs 66 million years ago, mammals would likely still be small, nocturnal creatures hiding from predators. Humans almost certainly would never have evolved. We owe our existence to a catastrophe.

14 🔬 Elaborate — Plants as Earth-Changers

14.1 How Plants Transformed Earth

When plants colonized land (~470 Mya), they didn’t just adapt to Earth — they remade it. Plants may be the most powerful geological force life has ever produced.

14.2 What Plants Did to Earth

Effect	Before Plants	After Plants
Atmosphere	O₂ ~15%	O₂ rose to 35% (Carboniferous)
Soil	Thin, rocky	Deep, organic-rich
Weathering	Slow (physical only)	Fast (roots + acid)
CO₂	Higher	Lower (plants absorb it)
Erosion	Rapid	Slower (roots stabilize)
Coal/Oil	None	Massive deposits from dead plants

carbonData = [
  {time: 0.5, co2: 7000, label: "Cambrian (no land plants)"},
  {time: 0.45, co2: 4500, label: "Ordovician"},
  {time: 0.4, co2: 3000, label: "First land plants"},
  {time: 0.35, co2: 2000, label: "Forests spreading"},
  {time: 0.3, co2: 300, label: "Carboniferous (peak forests)"},
  {time: 0.25, co2: 2000, label: "Permian extinction"},
  {time: 0.2, co2: 1500, label: "Triassic"},
  {time: 0.15, co2: 1200, label: "Jurassic"},
  {time: 0.1, co2: 1000, label: "Cretaceous"},
  {time: 0.05, co2: 500, label: "Paleogene"},
  {time: 0.0, co2: 420, label: "Today"}
]

Plot.plot({
  title: "Atmospheric CO₂ Over the Past 500 Million Years (ppm)",
  subtitle: "Land plants dramatically drew down CO₂ — until mass extinction reversed it",
  width: 700,
  height: 350,
  x: {label: "Billions of years ago", reverse: true, grid: true},
  y: {label: "CO₂ (ppm)", grid: true},
  marks: [
    Plot.areaY(carbonData, {x: "time", y: "co2", fill: "#fdcb6e", fillOpacity: 0.3, curve: "natural",
        tip: true}),
    Plot.line(carbonData, {x: "time", y: "co2", stroke: "#e17055", strokeWidth: 3, curve: "natural",
        tip: true}),
    Plot.dot(carbonData, {x: "time", y: "co2", fill: "#e17055", r: 4,
        tip: true}),
    Plot.ruleY([420], {stroke: "#d63031", strokeWidth: 2, strokeDasharray: "5,5"}),
    Plot.text([{x: 0.35, y: 550}], {x: "x", y: "y", text: d => "Today's CO₂ (420 ppm)", fill: "#d63031", fontSize: 10, fontWeight: "bold"})
  ]
})

14.2.1 💡 The Carboniferous Coal Connection

During the Carboniferous period (~360–300 Mya), vast swamp forests grew and died. The dead trees piled up because bacteria hadn’t yet evolved the ability to decompose wood (lignin). This buried carbon became coal. By removing so much CO₂, forests cooled Earth into an ice age.

This is the same coal we burn today for electricity — we’re releasing 300-million-year-old carbon back into the atmosphere.

15 ✅ Evaluate — Connecting It All

15.1 Applying Your Understanding

15.1.1 🧪 Evaluate Questions

1. Explain how photosynthetic organisms both created and destroyed life on early Earth (the Great Oxidation Event).

2. Construct a timeline showing at least 5 ways life changed Earth’s atmosphere, oceans, or surface over 4 billion years.

3. Argue whether mass extinctions have been “good” or “bad” for the evolution of complex life. Use evidence from at least 2 extinction events.

4. Compare the CO₂ levels during the Carboniferous with today. What caused the difference? Why should this concern us?

5. Predict: If all photosynthetic organisms died tomorrow, what would happen to Earth’s atmosphere within 1,000 years? 10,000 years?

15.1.2 📝 Model Update

Update your model of “What makes a planet habitable” with these new ideas:

• Life doesn’t just respond to its environment — it creates its environment
• Mass extinctions are destructive but also necessary for the evolution of complex life
• Earth’s atmosphere, temperature, and surface are all partly products of biological processes
• How does this change your thinking about whether other planets could develop life?