skip navigation
Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 1: Nature of Life on Earth

Key Ideas

Life is not easy to define.

Six basic properties.
-- Order, Response, Growth, Energy, Reproduction,, and Evolution

Evolution plays a key role in all aspects of life.

Carbon-based chemistry is central in biology.

Kinds of molecules:
-- Carbs, Lipids, Proteins, Nucleic Acids

Two kinds of cells: Eukaryotes and Prokaryotes

Metabolism and classification based on sources of energy and raw materials.

The structure of DNA codes operating instructions.

DNA informs the Tree of Life

There is evidence for a common ancestor.

What is Life?

We can identify six conditions

Growth and Development
Energy Utilization
Response to Environment
Evolutionary Adaptation


A overarching structure and/or organization to the component molecules and atoms

Growth and Development

The ability to grow; add mass and/or capabilities.

Utilizes Energy

Can harvest energy and use it to grow, react, develop, and counteract the tendency toward decay.

Response to Environment

Ability to sense and react to environment.


The ability to make copies itself.

Evolutionary Adaptation

Living organisms change, or evolve, over successive generations as they respond and adapt to the environment and other organisms.


Origin of the Theory of Evolution

Anaximander (c. 610-547 B.C.)
-- Life arose in water

Rise of Aristotelian Philosophy
-- Unchanging aesthetic

Jean-Baptiste Lemarck (1744-1829)
--Basic idea of passing traits onto future progeny
--Wrong mechanism

Charles Darwin (1809-1882)
Alfred Wallace (1823-1913)

Two Facts and a Conclusion

Fact 1: Any common group can produce far more offspring than the environment can support.
--Immediately implies competition.

Fact 2: Individual offspring vary in traits passed from parents to offspring.
--Assumes the existence of traits and heredity.

Conclusion: Some groups do better and these will be more likely to leave a larger number of offspring.

Beneficial traits are naturally selected.

Evidence for Evolution

Evidence in the diversity and similarities of species.

Artificial selection
--Domestic dogs

Observed evolution of species.
--Bacteria develop resistance to antibiotics

Carbon chemistry

Has 6 protons (and 6 neutrons)
Wants 6 electrons.
Two form a complete shell.
Four are available for bonds with other elements.

All Hail Carbon

Carbon is very versatile.
Able to form a vast variety of structures.
Carbon compounds dissolve in liquid
H+C = hydrocarbons

Carbon forms basis of cells

Cell is the basic unit of living organisms.
All living cells share similarities.

Kinds of molecules in cells

Cell structure
--Catalysts: Enzymes
--Composed of amino acids
--70 known, only ~20 used
--Left-handed only
Nucleic Acids

Role of cells

Concentrate molecules
Protection from environment

Two varieties of cells

Prokaryotes (no nucleus)
Eukaryotes (nucleus)

Metabolic Needs of Life

Source of raw materials (i.e. carbon)
-Heterotroph (hetero=other, troph=feed)
---Carbon from eating
-Autotroph (auto=self, troph=feed)
---Carbon from CO2

Source of energy (fuel metabolism)

--Chemical energy

All Hail Water

Allows organic chemicals to be readily available
Medium of transport
Liquid at temperatures where reactions are fast.
Used in many metabolic reactions
Appears to be necessary for all life on Earth


Complex molecule

Composed of four bases
--Adenine (A)
--Guanine (G)
--Thymine (T)
--Cytocine (C)

Operating manual

Used by all life

DNA Code

Contains sets of instructions
--Each set of instructions = gene

Entire set of genes = genome
--Humans have 3 billion bases
--20,000-25,000 genes

Every cell has complete genome

Three bases = word

DNA Replication and Mutations

Very accurate replication
One error per billion bases copied


Errors in replications form the basis of mutations
Most mutations are neutral, some are bad, but some are beneficial
Good mutations get propagated by natural selection

DNA and the Tree of Life

Scientist can determine, or sequence, the genomes of organisms on Earth and look for similarities.

Evidence for a common ancestor:
-All life has uses similar cells
-All life uses same 20 amino acids, while over 70 are known
-All life uses left-handed amino acids
-All life uses the same energy molecule ATP
-All life uses DNA
-Tree of life points toward common ancestor

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

[ Return to the Astronomy 141 Main Page | Unit 2 Page ]

Copyright Scott Gaudi All Rights Reserved.