What scientific models explain how life could have originated on Earth?
Topic 7.13 Origin of Life on Earth: describe the scientific models for the origin of life, including the RNA world and the evidence supporting them.
A focused answer to AP Biology Topic 7.13, covering models for the origin of life, the formation of organic monomers, the RNA world hypothesis, protocells, the geological timeline, and the evidence behind these models, with a worked example.
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What this topic is asking
The College Board (Topic 7.13) wants you to describe the scientific models for the origin of life on Earth, including the formation of organic monomers, the RNA world hypothesis, protocells, and the evidence supporting these models.
From simple molecules to monomers
Polymers, protocells and the RNA world
The evidence and timeline
Try this
Q1. State the order of the main stages proposed for the origin of life. [2 points]
- Cue. Simple molecules to organic monomers, monomers to polymers, polymers enclosed in membrane-bound protocells, leading to self-replicating systems and the first cells.
Q2. Explain why the RNA world hypothesis favors RNA as the first self-replicating molecule. [2 points]
- Cue. RNA can both store information and catalyze reactions (ribozymes), so a single RNA molecule could carry genetic information and copy itself, unlike DNA or protein alone.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)4 marksSection II (long FRQ excerpt). (a) Describe the sequence of stages proposed by scientific models for the origin of life, from simple molecules to the first cells. (b) Explain why the RNA world hypothesis proposes that RNA, rather than DNA or protein, was the first self-replicating molecule.Show worked answer →
A 4-point describe-and-explain FRQ on the origin of life.
(a) Describe (2 points): (1 point) simple inorganic molecules formed organic monomers (amino acids, nucleotides) under early-Earth conditions; (1 point) monomers joined into polymers, which became enclosed in membrane-bound protocells able to maintain an internal environment, eventually leading to self-replicating systems and the first cells.
(b) Explain (2 points): (1 point) RNA can both store genetic information (like DNA) and catalyze reactions (like an enzyme), as ribozymes do; (1 point) so a single RNA molecule could both carry information and copy itself, solving the chicken-and-egg problem of needing both genes and enzymes at once.
Markers reward the ordered sequence (monomers to polymers to protocells) and explaining RNA's dual role as the reason for the RNA-first model.
AP 2017 (style)1 marksSection I (multiple choice). The RNA world hypothesis is supported by the observation that some RNA molecules (ribozymes) can: (A) store no information. (B) catalyze chemical reactions. (C) only exist inside modern nuclei. (D) replicate DNA directly.Show worked answer →
The correct answer is (B).
Ribozymes are RNA molecules that catalyze reactions, showing that RNA can act as an enzyme as well as store information. This dual ability supports the idea that RNA could have been the first self-replicating molecule. (A), (C) and (D) are incorrect descriptions of ribozymes.
Related dot points
- Topic 7.7 Common Ancestry: describe the structural and molecular features shared by all organisms that indicate common ancestry.
A focused answer to AP Biology Topic 7.7, covering the shared features of all life (DNA, the genetic code, ribosomes, core metabolism, membranes) that indicate common ancestry, and how conserved features reveal deep relationships, with a worked example.
- Topic 7.6 Evidence of Evolution: describe the lines of evidence (fossil, anatomical, molecular, biogeographical) that support evolution.
A focused answer to AP Biology Topic 7.6, covering fossil, anatomical (homologous and vestigial structures), embryological, molecular and biogeographical evidence for evolution, with a worked interpretation of molecular data.
- Topic 7.9 Phylogeny: interpret and construct phylogenetic trees and cladograms from shared characters and molecular data.
A focused answer to AP Biology Topic 7.9, covering phylogenetic trees and cladograms, shared derived characters, nodes and common ancestors, out-groups, and reading relatedness from a tree, with a worked tree interpretation.
- Topic 1.6 Nucleic Acids: describe the structural similarities and differences between DNA and RNA and explain how the directionality and base pairing of nucleic acids support their function.
A focused answer to AP Biology Topic 1.6, covering nucleotide structure, the antiparallel double helix, base pairing, the 5' to 3' directionality, and the structural differences between DNA and RNA.
- Topic 2.11 Origins of Cell Compartmentalization: describe the similarities and differences in compartmentalization between prokaryotic and eukaryotic cells, and the evidence for the endosymbiotic origin of mitochondria and chloroplasts.
A focused answer to AP Biology Topic 2.11, covering the endosymbiotic theory, the evidence that mitochondria and chloroplasts descend from free-living prokaryotes, and the origin of the endomembrane system.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)