Campbell Biology Chapter 25 Review: The History of Life on Earth

25.1 Chemistry before life: how the first cells might have formed

pp. 507–510

On the AP exam? Yes

Topic 7.12 tests the timeline (Earth about 4.6 billion years old, too hostile for life until about 3.9 billion, oldest fossils about 3.5 billion), the idea that organic molecules could form without life on an oxygen-poor early Earth, and the RNA world hypothesis. You won't need to recall Miller and Urey's names or the details of particular experiments.

In the course: Topic 7.12 Origins of Life on Earth, Topic 1.6 Nucleic Acids (notes, videos and more questions)

Key points

Earth is roughly 4.6 billion years old. For its first few hundred million years, impacts from leftover space debris kept it too hot and violent for life, probably until around 3.9 billion years ago. The oldest microbe fossils date to roughly 3.5 billion years ago, so life most likely began somewhere in that window.
Scientists split the origin of life into steps they can test in a lab: small organic molecules form without any living thing; they link into chains (polymers); the chains get wrapped inside a membrane; and some molecule starts copying itself, so traits can be passed on.
In the 1920s, Oparin and Haldane proposed that an early atmosphere with almost no free oxygen, fed energy by lightning and ultraviolet light, could build organic molecules from simple gases. In 1953, Miller and Urey ran sparks through a flask of gases and water and got amino acids. Later runs with other gas mixes, including volcano-like ones, also made organic molecules.
Some building blocks may have come from space. Certain meteorites carry amino acids, and samples brought back from the asteroid Bennu contain amino acids and all five bases found in DNA and RNA.
Monomers can join into short chains without enzymes, for example when a solution of them dries out on hot rock or mineral surfaces. Lipids in water also assemble on their own into membrane-bound bubbles (vesicles), which can grow, split and hold a chemistry inside that differs from the water outside.
The RNA world hypothesis says RNA was the first genetic material. RNA can store information, since its bases pair, and some RNA molecules (ribozymes) can speed up reactions, including copying short RNA strands. No genetically coded proteins were needed.
Once molecules could copy themselves with small errors, sequences that lasted longer or copied faster left more copies: natural selection at the level of molecules. DNA probably took over long-term storage later, since its two strands make it sturdier and it's copied more faithfully, while RNA kept its role as the messenger between genes and proteins.

Key terms (8)

abiotic synthesis: Making a molecule without the help of any living thing, for example amino acids forming from gases when energy is added.
reducing atmosphere: An atmosphere with little or no free oxygen that tends to donate electrons, which makes it easier for simple gases to combine into organic molecules.
Miller–Urey experiment: A 1953 lab test that sent sparks through a sealed mix of gases and water to imitate early Earth. Amino acids formed, showing life's building blocks can arise without life.
polymer: A long molecule made of many similar units (monomers) joined together, such as a protein or a strand of RNA.
vesicle: A tiny fluid-filled bubble wrapped in a lipid bilayer. Vesicles can form on their own when lipids are added to water.
protocell: A hypothetical early stage on the way to a cell: a membrane bubble holding molecules, with a chemistry inside that differs from its surroundings.
ribozyme: An RNA molecule that acts as a catalyst. It's like an enzyme, but made of RNA instead of protein.
RNA world hypothesis: The idea that early life used RNA both to carry genetic information and to catalyze reactions, before DNA and proteins took over those jobs.

Check yourself: 25.1 Chemistry before life: how the first cells might have formed

4 questions on 25.1 Chemistry before life: how the first cells might have formed. Pick an answer to see if you got it, and why.

Question 1 of 4

Geologists date the oldest minerals on Earth, the end of a period of heavy impacts from space, and the oldest known microbial fossils. Taken together, which statement about when life began do these dates support?

Question 2 of 4

Which property of RNA best explains why many scientists think it, rather than DNA or protein, was the first genetic material?

Question 3 of 4

Question 4 of 4

In a model protocell, the lipid membrane lets single nucleotides pass in from the surrounding water but keeps long RNA strands from leaking out. Why would this property matter for the evolution of the first cells?

0 of 4 answered

25.2 Reading and dating the fossil record

pp. 510–514

On the AP exam? Yes

Topic 7.6 tests how fossils are dated (rock layers, isotope decay such as carbon-14) and how they show change over time. You may need a quick half-life calculation; you won't need the names of the fossil skull bones.

In the course: Topic 7.6 Evidence of Evolution (notes, videos and more questions)

Key points

Most fossils are found in sedimentary rock, which builds up in layers called strata. In an undisturbed stack, lower layers are older, so the order of the layers tells you which fossils came first. That's a relative age, not an age in years.
The fossil record is incomplete and uneven. Organisms with shells, bones or wood, that were common and widespread, that lasted a long time as species, and that lived where sediment piles up are much more likely to be preserved and found. New discoveries keep filling in gaps.
Radiometric dating gives an age in years. A radioactive parent isotope turns into a daughter isotope at a steady rate, described by its half-life: the time for half of the parent to decay. Heat, pressure and chemistry don't change it.
After one half-life, 1/2 of the parent is left; after two, 1/4; after three, 1/8. Comparing how much parent is left (or the daughter-to-parent ratio) with the starting amount tells you how many half-lives have passed.
Carbon-14 (half-life 5,730 years) dates once-living material up to roughly 50,000 years old, by comparing its ¹⁴C to its ¹²C. Older fossils are usually dated indirectly: geologists date volcanic layers above and below them using slow-decaying isotopes like uranium-238 or potassium-40.
Fossils can show a new group taking shape step by step. Fossils linking small meat-eating dinosaurs to birds show feathers, a wishbone and hollow bones appearing in ground-dwelling dinosaurs long before the full set of flight features evolved.

Key terms (9)

macroevolution: Large-scale evolution above the level of one species, such as the rise of a new group or a mass extinction.
fossil record: All the fossils found so far, arranged by where they sit in the rock layers and by their ages.
sedimentary rock: Rock formed as sand, mud or other particles settle in layers and harden. It's where most fossils are found.
strata: The layers of sedimentary rock. In an undisturbed stack, deeper layers formed earlier.
relative age: Whether a fossil is older or younger than another, judged from its position in the rock layers, without a number of years.
radiometric dating: Finding an age in years by measuring how much of a radioactive isotope has decayed into its daughter product.
half-life: The time it takes for half of a radioactive isotope in a sample to decay. Each isotope's half-life is fixed.
transitional fossil: A fossil with a mix of features from an older group and a newer group that descended from it, showing a change partway done.
tetrapod: A vertebrate with four limbs, or descended from four-limbed ancestors: amphibians, reptiles (including birds) and mammals.

Check yourself: 25.2 Reading and dating the fossil record

4 questions on 25.2 Reading and dating the fossil record. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A museum visitor asks why the scientists didn't use carbon-14 to date a 70-million-year-old dinosaur bone. Which answer is best?

Question 2 of 4

Which statement correctly contrasts relative dating with radiometric dating?

Question 3 of 4

A cliff shows undisturbed rock layers. From the bottom up they are: sandstone containing fossil species X; a volcanic ash layer dated to 310 million years; shale containing fossil species Y; a volcanic ash layer dated to 296 million years; and limestone containing fossil species Z. Which conclusion about fossil Z is justified?

Question 4 of 4

In one quarry, collectors have found about 900 fossil clam shells and only 2 impressions of jellyfish from the same layers. Which inference is most justified?

0 of 4 answered

25.3 Big milestones: oxygen, eukaryotes, animals and land

pp. 514–519

On the AP exam? Yes

Endosymbiosis (Topic 2.10), cyanobacteria oxygenating the air (Topic 3.4) and the age of the oldest fossils (Topic 7.12) are tested. The eon and era names, the Ediacaran biota, snowball Earth and the Cambrian explosion are background.

In the course: Topic 2.10 Origins of Cell Compartmentalization, Topic 3.4 Photosynthesis, Topic 7.12 Origins of Life on Earth, Topic 7.6 Evidence of Evolution (notes, videos and more questions)

Key points

Geologists divide Earth's history into three eons: the Archaean, the Proterozoic and the Phanerozoic (the last roughly 540 million years). The Phanerozoic is split into the Paleozoic, Mesozoic and Cenozoic eras, and the boundaries between eras line up with mass extinctions.
The first life was prokaryotic. The oldest widely accepted evidence is roughly 3.5 billion years old: stromatolites, layered rocks built by mats of microbes that trap and bind sediment. For well over a billion years, prokaryotes were the only life on Earth.
The oxygen in our air comes from photosynthesis that splits water, which first evolved in cyanobacteria-like prokaryotes. Early O₂ reacted with iron dissolved in seawater and settled out as rusty layers (banded iron formations). Only later did O₂ build up in the air, jumping sharply in the Great Oxidation Event around 2.4 billion years ago.
Rising oxygen harmed many anaerobic prokaryotes, which died out or survived only in oxygen-free places like deep mud. It also opened the way for aerobic respiration, which gets far more ATP from food.
Eukaryotes came later; the oldest fossils most scientists accept are roughly 1.6–1.8 billion years old. By endosymbiotic theory, mitochondria descend from aerobic bacteria taken in by a host cell (most likely related to a group of archaea), and chloroplasts later from cyanobacteria taken in by a cell that already had mitochondria. Both organelles have two membranes, their own circular DNA, bacteria-like ribosomes, and divide by splitting.
Multicellular eukaryotes date back more than a billion years, but large, varied soft-bodied ones (the Ediacaran biota) show up only after a run of nearly global ice ages ended. Then, beginning about 539 million years ago, fossils of many animal phyla appear in a geologically short burst, the Cambrian explosion, along with the first big predators, spines and armor. DNA evidence suggests many animal lineages had split earlier.
Plants, fungi and animals began moving onto land roughly 500–470 million years ago. Land life needed ways to avoid drying out and to reproduce out of water; early plants did it in partnership with fungi. Arthropods came ashore early, four-limbed vertebrates by about 370 million years ago, and our own species only about 300,000 years ago.

Key terms (10)

geologic record: The timeline of Earth's history, divided into eons, eras and periods based on rock layers and fossils.
stromatolite: A layered rock built up by mats of microbes, often cyanobacteria, that trap sediment. Fossil ones are the oldest widely accepted signs of life.
cyanobacteria: Photosynthetic bacteria that split water and release O₂. Their ancestors filled Earth's air with oxygen, and one was taken in to become the first chloroplast.
Great Oxidation Event: The sharp rise in oxygen in Earth's atmosphere around 2.4 billion years ago, caused by photosynthetic prokaryotes. Also called the oxygen revolution.
banded iron formation: Rock with red, iron-rich layers that formed when oxygen from photosynthesis reacted with iron dissolved in the oceans.
endosymbiotic theory: The idea that mitochondria and chloroplasts were once free-living prokaryotes that were taken in by a host cell and stayed, becoming organelles.
endosymbiont: A cell that lives inside another cell (its host).
Ediacaran biota: A group of large, soft-bodied multicellular organisms found in rocks from just before the Cambrian period, roughly 575–539 million years ago.
Cambrian explosion: A burst of animal evolution starting about 539 million years ago, when fossils of many animal phyla, including hard-bodied predators and prey, appear quickly.
snowball Earth: The hypothesis that ice covered nearly all of Earth during several ice ages late in the Proterozoic, which may have held back large multicellular life.

Check yourself: 25.3 Big milestones: oxygen, eukaryotes, animals and land

4 questions on 25.3 Big milestones: oxygen, eukaryotes, animals and land. Pick an answer to see if you got it, and why.

Question 1 of 4

A certain antibiotic blocks bacterial ribosomes but doesn't affect the ribosomes in the cytoplasm of human cells. Researchers find that in human cells it also slows protein synthesis inside mitochondria. How does this finding fit endosymbiotic theory?

Question 2 of 4

Based on current evidence, which pairing best describes the partners that formed the first eukaryotic cell with a mitochondrion?

Question 3 of 4

Students grow cyanobacteria in a sealed, lit tank of water rich in dissolved iron (Fe²⁺). For the first 20 days, rust-colored particles settle to the bottom and no oxygen gas is detected in the air above the water. After day 25, oxygen begins to build up in the air above the water. What best explains the delay?

Question 4 of 4

Geologists find 3.4-billion-year-old rock mounds made of thin, wavy layers. Similar layered mounds can also form from minerals settling out of water, with no life involved. Which additional finding would best support the claim that microbes built these mounds?

0 of 4 answered

25.4 Drifting continents, mass extinctions and adaptive radiations

pp. 519–524

On the AP exam? Yes

Topic 7.10 tests adaptive radiation and speciation rates, and Topic 8.7 uses continental drift and the asteroid that ended the dinosaurs as examples of geological events reshaping life. You don't need to memorize the big five extinctions, their dates or the plate names.

In the course: Topic 7.10 Speciation, Topic 8.7 Disruptions in Ecosystems, Topic 7.11 Variations in Populations (notes, videos and more questions)

Key points

Whether a group of organisms grows or shrinks depends on two rates. If new species form faster than old ones go extinct, the group gains species; if extinction outpaces speciation, it declines. It works like births and deaths in a population.
Earth's crust is broken into plates that ride on the hot mantle beneath and move a few centimeters a year (continental drift). Where plates pull apart, slide past each other or collide, you get new sea floor, earthquakes, volcanoes and mountains.
Continental drift shapes life in three main ways. Moving land changes climate and habitats: when nearly all land joined into the supercontinent Pangaea by about 250 million years ago, coastlines and sea levels shifted and the interior turned harsh. Splitting land apart separates populations, so allopatric speciation can happen on a continental scale. And drift explains why related species or matching fossils turn up on landmasses that are now far apart.
Extinction happens all the time at a low background rate. A mass extinction is a short burst when a large share of species around the world disappears. The fossil record shows five big ones in the last 500 million years, each wiping out at least half of marine species.
The end-Permian extinction, about 252 million years ago, was the worst, killing roughly 90% of marine species. It's linked to huge volcanic eruptions in Siberia that warmed the climate and left oceans low in oxygen. The end-Cretaceous extinction, 66 million years ago, ended the non-bird dinosaurs; a worldwide clay layer rich in iridium and the Chicxulub crater in Mexico point to an asteroid about 10 km wide.
Recovery takes millions of years, and lineages that vanish never come back. Survivors often undergo adaptive radiation, quickly splitting into many species that fill emptied niches, as mammals did after the dinosaurs. Radiations also follow a new key trait or arrival on remote islands with few competitors.
Today, habitat loss, climate change and other human activities are pushing extinction rates to tens to hundreds of times the background rate. Many scientists warn that a sixth mass extinction could follow if trends continue.

Key terms (10)

plate tectonics: The theory that Earth's crust is made of large plates that move slowly over the hot mantle beneath them.
continental drift: The slow movement of continents over time as the plates carrying them shift.
Pangaea: The supercontinent that formed when nearly all of Earth's landmasses joined, by about 250 million years ago. It later broke apart.
background extinction: The normal, steady rate at which species go extinct between mass extinctions.
mass extinction: A geologically brief event in which a large fraction of species around the world go extinct, far above the background rate.
end-Permian extinction: The largest mass extinction, about 252 million years ago, marking the end of the Paleozoic era. It wiped out roughly 90% of marine species.
end-Cretaceous extinction: The mass extinction about 66 million years ago, likely triggered by an asteroid impact, that ended the non-bird dinosaurs and the Mesozoic era.
adaptive radiation: A burst of speciation in which one lineage gives rise to many species suited to different ways of life.
niche: An organism's role in its community: what it eats, where it lives and how it interacts with other species.
endemic species: A species found naturally in only one place, such as a single island chain.

Check yourself: 25.4 Drifting continents, mass extinctions and adaptive radiations

4 questions on 25.4 Drifting continents, mass extinctions and adaptive radiations. Pick an answer to see if you got it, and why.

Question 1 of 4

Paleontologists estimate speciation and extinction rates for a group of marine snails in four time intervals (invented data). Interval (million years ago) | New species per million years | Extinctions per million years 80–70 | 4.2 | 2.1 70–60 | 3.0 | 3.5 60–50 | 1.5 | 1.4 50–40 | 2.8 | 1.0 In which interval was the number of species in this group most likely falling?

Question 2 of 4

Two continents that have been apart for 40 million years, each with its own land mammals, drift together and become joined by land. Which outcome over the next few million years is most likely?

Question 3 of 4

Rocks on an island that now lies at 70° north hold fossil palm trees about 200 million years old. One hypothesis says the island was near the equator then and has drifted north. Another says the whole planet was warm enough for palms near the poles. Which finding would best support the drift hypothesis?

Question 4 of 4

Which observation would most strongly support the hypothesis that an asteroid impact caused the end-Cretaceous mass extinction?

0 of 4 answered

25.5 Developmental genes and new body plans

pp. 525–529

On the AP exam? Background

The current course doesn't name evo-devo, heterochrony or paedomorphosis, but Topics 6.5 and 6.6 test the idea underneath: when and where genes are switched on shapes the phenotype, and Hox genes come up as an example.

In the course: Topic 6.5 Regulation of Gene Expression, Topic 6.6 Gene Expression and Cell Specialization, Topic 6.7 Mutations (notes, videos and more questions)

Key points

Evo-devo studies how changes in the genes that guide development lead to differences in body form between species. A small genetic change can have a big effect when it alters how fast, when or where these genes act.
Heterochrony is an evolved shift in how fast, or when, a developmental step happens. If one body part grows faster or for longer, the adult shape changes. A giraffe has the same seven neck bones you do; each one simply grows much longer.
Paedomorphosis is one kind of heterochrony: if the reproductive organs mature early relative to the rest of the body, adults keep traits that were juvenile in their ancestors, such as a larval body shape, external gills or a lack of wings.
Changes in spatial pattern matter too. Homeotic genes control where body parts form. Hox genes are a family of homeotic genes that code for transcription factors telling cells along an animal's head-to-tail axis what to become. Shifting where a Hox gene is expressed can move, remove or convert body parts. MADS-box genes play a similar role in flowers.
Many developmental genes are ancient and shared across animals. New forms can come from gene duplication, where a spare copy is free to take on a new role, or from changes in a gene's coding sequence that alter its protein.
A change to a gene's coding sequence alters the protein everywhere it's made, which is often harmful. A change to a regulatory sequence such as an enhancer can switch the gene off or on in just one tissue. That's why many changes in body form trace back to gene regulation rather than to changes in the protein itself.

Key terms (9)

evo-devo: Evolutionary developmental biology: the study of how changes in developmental genes produce new body forms.
developmental gene: A gene whose product guides how an embryo grows and where its parts form, often a transcription factor or a signaling molecule.
heterochrony: A shift, over many generations, in how fast or when parts of an embryo develop, such as one body part growing for longer.
paedomorphosis: Keeping juvenile features into adulthood because reproductive development speeds up relative to the rest of the body.
homeotic gene: A master control gene that decides which body parts form where, such as which segment grows legs.
Hox genes: A family of homeotic genes in animals that code for transcription factors, telling cells along the head-to-tail axis what to become.
MADS-box genes: A family of plant genes that control which flower parts (sepals, petals, stamens, carpels) form in each ring of a flower.
enhancer: A stretch of DNA that transcription factors bind to turn up a gene's transcription, often in a specific tissue or at a specific time.
gene duplication: A mistake that leaves an extra copy of a gene. The spare copy can change and take on a new job while the original keeps the old one.

Check yourself: 25.5 Developmental genes and new body plans

4 questions on 25.5 Developmental genes and new body plans. Pick an answer to see if you got it, and why.

Question 1 of 4

Two closely related species of shrew-like mammals have identical genes for the proteins that build their snouts. In one species, snout growth continues about three weeks longer during development, producing a much longer snout. Which term best describes this kind of evolutionary change?

Question 2 of 4

In one firefly species, adult females are wingless and look much like the species' larvae, yet they mate and lay eggs. Males of the species and adult females of related species have wings. Which developmental change best explains these females?

Question 3 of 4

In the embryo of a segmented marine worm, Hox gene P is normally switched on in segments 11–20, which grow paddle-shaped swimming lobes; segments 1–10 grow feathery gill tufts. In a mutant line, a mutation in its regulatory DNA switches gene P on in segments 6–20 instead. What is the most likely result?

Question 4 of 4

Two populations of a seed-eating bird differ in beak depth. Gene B affects beak growth, and its protein-coding sequence is identical in both. Researchers attach the enhancer next to gene B from each population to a gene for a glowing protein and insert each version into bird embryos (invented data). Enhancer source | Glow in beak tissue | Glow in other tissues Deep-beaked population | strong | weak Shallow-beaked population | faint | weak What do these results best support?

0 of 4 answered

25.6 Old parts, new uses: how novelties and trends arise

pp. 529–531

On the AP exam? Background

Exaptation and species selection aren't named in the current course, but the main lesson is tested: mutations aren't directed by need (Topic 8.7), and selection only acts on present variation (Topic 7.2). Convergent evolution is in Topic 7.10.

In the course: Topic 8.7 Disruptions in Ecosystems, Topic 7.2 Natural Selection, Topic 7.10 Speciation, Topic 7.9 Phylogeny (notes, videos and more questions)

Key points

Evolution doesn't design from scratch. New forms arise by reworking structures and genes that already exist, so even a new body plan is built from modified older parts.
A complex organ can evolve in small steps as long as each step helps its owner at the time. A simple light-sensing spot already helps an animal tell light from dark, a cupped spot adds direction, and a lens adds a sharp image. Complex eyes evolved separately in several animal groups this way.
An exaptation is a structure that evolved for one use and was later put to another. Feathers probably first helped dinosaurs stay warm or show off, and only later were used for flight. Selection can't plan ahead; a trait spreads because of what it does for its owner now, not because of a use it might have someday.
A fossil 'trend', like a lineage steadily getting bigger, can be misleading if you follow only the branch that leads to a living species. Most lineages branch like a bush, and many side branches went other ways or died out.
Real trends can come from natural selection as an environment changes, or from species selection: lineages whose species last longer and split more often come to dominate, much like individuals that leave more offspring.
A trend has no built-in target. It lasts only as long as the conditions that favor it, so it can stall or turn around when the environment shifts. Mutations happen at random with respect to an organism's needs, and selection sorts whatever variation exists in the present environment.

Key terms (7)

descent with modification: Darwin's phrase for evolution: species descend from ancestors and change over generations, keeping many of the ancestors' features.
evolutionary novelty: A new structure or ability in a lineage, such as feathers or a turtle's shell, usually built by modifying older parts.
exaptation: A trait that evolved for one function and was later used for a different one.
convergent evolution: Unrelated lineages independently evolving similar traits because they face similar challenges.
evolutionary trend: A long-term, directional change in a lineage, such as increasing body size. It lasts only while conditions keep favoring it.
species selection: The idea that lineages whose species survive longer and form new species more often come to dominate over long periods.
random mutation: A mutation that happens regardless of whether it would help. The environment doesn't cause the mutations an organism needs.

Check yourself: 25.6 Old parts, new uses: how novelties and trends arise

4 questions on 25.6 Old parts, new uses: how novelties and trends arise. Pick an answer to see if you got it, and why.

Question 1 of 4

In an early group of beetles, males grew short horns that were used to dig burrows. In a later group descended from them, the horns are long and males use them mainly to fight rivals for mates. Which term best describes the horns' change in role?

Question 2 of 4

A student writes: "Fish that moved into dark caves lost their eyes because they no longer needed them, so their bodies stopped making eyes." Which correction is most accurate?

Question 3 of 4

A museum display arranges five fossil species in a straight line, from a small ancestor to a living giant tortoise, labeled "the march toward giant size." The full fossil record of the group includes 30 species, many of which stayed small or went extinct. What is the best critique of the display?

Question 4 of 4

Two lineages of sea snails start with 10 species each (invented data). Lineage X forms 0.30 new species per species per million years and loses 0.10. Lineage Y forms 0.20 and loses 0.15. If these rates hold, what is most likely after 10 million years?

0 of 4 answered