Great Ideas in Science: Lecture 13 – Evolving systems Professor Robert Hazen PROV 301 Life originated on Earth billions of years ago as a single cell, and has been evolving by the process of natural selection ever since.

Two Stages of Evolution Chemical Evolution – Lab experiments show complex molecules arise from simple ones Natural Selection – Once a reproducing cell exists, complexity arises from competition

The Origin of Life: Four Possibilities A miracle – an act of divine intervention An event consistent with chemistry and physics, but extremely unlikely An inevitable consequence of chemistry, given an appropriate environment and sufficient time Intelligent design

Chemical Evolution Life arose by a natural process of “emergent complexity,” consistent with natural laws. This hypothesis predicts that life began as a sequence of chemical steps.

Intelligent Design Life is “irreducibly complex.” Therefore, a supernatural designer must have formed it. This hypothesis requires a combination of natural and supernatural processes.

THE “DEBATE” “Both sides ought to be properly taught ... so people can understand what the debate is about.” G. W. Bush “Intelligent design should not be taught in high school biology classes as an alternative to evolution.” American Chemical Society

How Should Science Respond to ID? Design a research program that demonstrates the natural transition from chemical simplicity to emergent complexity. If biological complexity can be shown to arise spontaneously as the result of natural processes, then ID is unnecessary.

STONEHENGE

What is Emergent Complexity? Emergent phenomena arise from interactions among numerous individual particles, or “agents.”

The Emergence of Slime Mold  Chemical Potential Gradients Dictyostelium

The Emergence of Slime Mold Dictyostelium

Emergent Phenomena – Life

Central Assumptions of Origin-of-Life Research The first life forms were carbon-based. Life’s origin was a chemical process that relied on water, air, and rock. Life’s origin required a sequence of emergent steps of increasing complexity.

Four Emergent Steps Emergence of biomolecules Emergence of organized molecular systems Emergence of self-replicating molecular systems Emergence of natural selection

Emergence of Biomolecules The strategy is to use simple molecules to build larger molecules.

The Miller-Urey Experiment Organic synthesis near the ocean-atmosphere interface.

Organic Synthesis in Interstellar “Dense” Molecular Clouds Experiments at NASA Ames simulate this environment.

The Hydrothermal Hypothesis A “BLACK SMOKER”

Deep-Sea Vents Reactants: Pyruvic acid + CO2 + H2O Conditions: 200oC 2,000 atm 2 hours Products: A diverse suite of organic molecules

Self-Assembling Amphiphile Molecules

Self-Assembling Amphiphile Molecules

Minerals and Selection Mineral surfaces select amino acids

The Emergence of Self-Replicating Molecular Cycles The abiotic synthesis of such a “metabolic” cycle represents a “Holy Grail” for our experimental program.

The Emergence of Self-Replicating Molecular Cycles Prof. Harold J. Morowitz

The Emergence of Natural Selection The emergence of natural selection appears to be inevitable in any system of molecules that makes copies of itself.

CONCLUSIONS The origin of life on Earth is best understood in terms of a sequence of emergent chemical events. Each step added a degree of structure and complexity to the prebiotic world. No compelling evidence exists to suggest that life’s origin was other than a natural process.

Chemical Evolution: The Window of Opportunity Great Bombardment: Ended ~4 billion years ago Oldest Fossils: Evidence of life 3.7 billion years ago The first cell probably arose 4.0-3.5 billion years ago

Three Definitions of Evolution Evolution as Change: Life on Earth has changed over billions of years. Common Descent: All living things on Earth descended from a common ancestor. Natural Selection: The process by which life evolved is Darwinian natural selection

First Definition of Evolution: Change Over Time Observational evidence overwhelmingly supports the theory that life originated on Earth billions of years ago as a single cell, and has been changing ever since. Fossils Molecular Biology Cellular biology Comparative anatomy Observations of nature and of breeding

Observational Evidence for an Ancient Earth Annual events: Tree rings 15,000 years Varve deposits >500,000 years Ice cores 1,200,000 years Radiometric dating 4.5 x 109 years Geological Processes 109 years Astronomical data 1010 years

Half-Life The average time for decay of ½ of a batch of radioactive isotopes Wide range of half-lives Radon-222: 3.8 days Carbon-14: ~5,700 years Uranium-234: 250,000 years Uranium-238: 4.5 billion years

Life has Changed over Time: The Fossil Record Fossils are any evidence of past life. They include tracks and trails. Most commonly an organism’s hard parts: Bones, teeth, and shells are preserved in rock. Hard parts are often replaced by minerals. The Fossil Record is all fossils found, catalogued, & analyzed. Taken together, they show transitions. But the fossil record is very incomplete.

Life has Changed over Time: The Fossil Record A few examples: Horn coral – 500 million years old Trilobite – 350 million years old Ammonites – 200 million years old Primitive shark tooth – 100 million years old Whale bone – 15 million years old

Fossils Strata Younger = higher

Extinct Animals

TRILOBITES If you accept observational evidence, then the unambiguous conclusion is that life has changed over time.

Calvert Cliffs 15,000,000 m.y. If you accept observational evidence, then the unambiguous conclusion is that life has changed over time.

Fossils Fossils prove that life on Earth has changed over time; most are extinct. Fossils demonstrate that these changes are gradual and progressive. All known fossils fit into a pattern of continuous evolution—simple to complex.

The Story of Life First cell Eukaryotes Colonies Hard shells (the Cambrian explosion) Vertebrates Primates Humans

Geological Time

Evidence from Anatomy: Vestigial Features Adaptive features—modified functions. Vestigial features—familiar structures with no useful function. Human appendix Hind legs of whales

Evidence from Anatomy: Homologous Structures Compare forelimbs of vertebrates

How Did Evolution Occur? Natural Selection Populations exhibit variations. More individuals are born than will survive. The most fit individuals are more likely to reproduce.