Biology · 11th Grade · Evolution and the History of Life · Weeks 10-18

Early Earth and Origin of Life

Explores hypotheses about the conditions on early Earth and the scientific theories regarding the abiotic synthesis of organic molecules and the first cells.

• Common Core State Standards
• HS-LS4-1

About This Topic

Evidence for Evolution introduces the scientific foundations of evolutionary theory. Students evaluate multiple lines of evidence, including the fossil record, comparative anatomy (homologous and vestigial structures), embryology, and molecular biology (DNA and protein sequences). This topic is the cornerstone of HS-LS4-1, which requires students to communicate scientific information that common ancestry and biological evolution are supported by multiple lines of empirical evidence.

Students explore how biogeography explains the distribution of species and how modern genomic data has reinforced findings from the fossil record. This topic comes alive when students can physically compare skeletal structures or use digital databases to find similarities in the DNA of seemingly unrelated species, fostering a sense of discovery and connection across the tree of life.

Key Questions

1. Explain the Miller-Urey experiment and its significance in understanding the origin of life.
2. Analyze the 'RNA World' hypothesis as a potential pathway to the first genetic material.
3. Differentiate between the scientific theories for the origin of life and religious explanations.

Learning Objectives

• Explain the conditions proposed for early Earth that may have supported the origin of life.
• Analyze the Miller-Urey experiment and its contribution to understanding the abiotic synthesis of organic molecules.
• Evaluate the 'RNA World' hypothesis, explaining its role as a potential precursor to DNA-based life.
• Differentiate between scientific hypotheses for the origin of life and non-scientific explanations.

Before You Start

Chemical Bonding and Molecular Structure

Why: Students need to understand how atoms form molecules, particularly organic molecules like amino acids, to grasp abiotic synthesis.

Properties of Water

Why: Understanding water's role as a solvent and its presence in early Earth's oceans is crucial for understanding where life may have originated.

Key Vocabulary

Abiotic Synthesis	The process by which organic molecules, the building blocks of life, are formed from inorganic substances in the absence of life.
Protocell	A self-organized, spherical collection of lipids proposed as a stepping stone toward the origin of life, exhibiting some characteristics of living cells.
Miller-Urey Experiment	An experiment simulating early Earth conditions that produced amino acids and other organic compounds from inorganic precursors, supporting the idea of abiotic synthesis.
RNA World Hypothesis	The theory that RNA, not DNA, was the primary form of genetic material for early life, due to RNA's catalytic and information-carrying capabilities.

Watch Out for These Misconceptions

Common Misconception

Students often think that 'evolution is just a theory' means it is an unproven guess.

What to Teach Instead

In science, a 'theory' is a well-substantiated explanation based on a body of facts. Comparing the 'Theory of Evolution' to the 'Theory of Gravity' in a structured discussion helps students understand the weight of scientific evidence.

Common Misconception

Many believe that humans evolved from modern monkeys.

What to Teach Instead

Evolutionary theory states that humans and modern monkeys share a common ancestor, not that one turned into the other. Using a 'family tree' analogy during a cladogram activity helps clarify this branching relationship.

Active Learning Ideas

Inquiry Circle: The Great Bone Sort

Groups are given sets of images or models of vertebrate forelimbs (human, whale, bat, cat). They must identify homologous structures and discuss how these similarities point to a common ancestor despite different functions.

40 min·Small Groups

Gallery Walk: Transitional Fossils

Stations feature 'mystery' fossils like Tiktaalik or Archaeopteryx. Students examine the features of each and place them on a timeline, explaining which groups of animals they appear to link together.

35 min·Pairs

Think-Pair-Share: Molecular Evidence

Students compare the amino acid sequences of Cytochrome C across different species. They calculate the percentage of similarity and discuss in pairs why a chimpanzee's sequence is more similar to a human's than a yeast's sequence is.

25 min·Pairs

Real-World Connections

• Astrobiologists at NASA's Jet Propulsion Laboratory analyze data from Mars rovers, searching for organic molecules and evidence of past water, which could indicate conditions suitable for life's origin.
• Researchers in synthetic biology attempt to create artificial cells or protocells in laboratories, drawing inspiration from early Earth conditions to understand the fundamental requirements for life.

Assessment Ideas

Quick Check

Present students with a diagram of the Miller-Urey apparatus. Ask them to label the components representing early Earth's atmosphere, ocean, and energy source, and write one sentence explaining what the experiment demonstrated.

Discussion Prompt

Pose the question: 'If we found evidence of simple organic molecules on another planet, what would that suggest about the possibility of life originating there?' Facilitate a class discussion connecting this to abiotic synthesis and the conditions on early Earth.

Exit Ticket

Ask students to write two sentences explaining the main idea behind the 'RNA World' hypothesis and one reason why it is considered a significant step in understanding the origin of life.

Frequently Asked Questions

What are homologous structures?

Homologous structures are physical features in different species that are similar because they were inherited from a common ancestor. An example is the similar bone structure in the wings of a bird and the flippers of a whale.

How does the fossil record support evolution?

The fossil record provides a chronological 'snapshot' of the past, showing how species have changed over millions of years and documenting the existence of transitional forms that bridge the gap between major groups.

How can active learning help students understand evolutionary evidence?

Active learning, such as analyzing real DNA sequences or handling fossil casts, allows students to act as scientists. By finding the evidence themselves rather than just reading about it, they build a much deeper understanding of how multiple independent fields of study all point to the same conclusion.

What is a vestigial structure?

A vestigial structure is a 'leftover' feature that served a purpose in an ancestor but is no longer functional in the modern species, such as the pelvic bones in whales or the human appendix.