“There is grandeur in this view of life” Natural selection and the complexity of life Need some help?
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(Duane D220) Earth is the most remarkable planet yet known
Scientists estimate there are at least 10 million species alive today.
We have yet to describe 86% of them. An ancient debate
“[A god created life because] without them the universe will be incomplete, for it will not contain every kind of animal which it ought to contain, if it is to be perfect.”
- Plato
“[When] everything turned out as it would have if it were on purpose, there the creatures survived, being accidentally compounded in a suitable way.”
- Empedocles
Greek, Chinese, Roman, and Islamic philosophies, among others, had notions of life as a mutable, changing phenomenon. Our daily dose of Aristotle
Aristotle’s History of Animals was the earliest surviving zoological work:
Book 1: The grouping of animals and the parts of the human body
Book 2-3: The organs of vertebrates
Book 4: Invertebrates
Book 5-7: Reproduction of animals and humans
Book 8-9: The behaviors of animals Aristotle rejected Empedocles and embraced the ideas of Plato. He would be the prime influence on Western science for 2000 years. Not so static after all
Selective breeding showed that organisms could change form dramatically over just hundreds of years. Buried monsters
Fossils refute the notion that the Earth has remained static over its history. Darwin finds the evidence Natural selection
A simple equation: Variation + DIfferential Survival = Evolution
Living species
Extinct species
There are likely billions of extinct species. Natural selection
The basic mechanism:
1. Life arises 2. As reproduction happens, slight variations occur through random chance 3. Some variations help an organism survive, others do not 4. Organisms with detrimental traits are less likely to reproduce 5. As variation accumulates, groups of organisms become distinct
Three key facts:
1. All life is constantly evolving 2. Evolution by natural selection is slow 3. Evolution does not always increase complexity The origin of life
Most common elements by mass:
Human Earth’s Earth’s Earth’s Universe body crust oceans atmosphere
Oxygen Oxygen Oxygen Nitrogen Hydrogen
Carbon Magnesium Hydrogen Oxygen Helium
Hydrogen Silicon Chlorine Argon Oxygen 96% of Nitrogen Iron Sodium Carbon Carbon human body
The origin of life is one of the great unsolved mysteries of science. Two possible origins
Endogenous Exogenous
Life arose on Earth Life arose elsewhere in the Universe
● All the ingredients are right here ● Sometimes called panspermia ● Life appears very quickly after the ● We observe material transfer Earth is capable of supporting it between the planets Early quirks
Homochirality Amino acids
Chirality is the handedness of a Amino acids are basic organic chemical molecule. Chiral molecules are mirror compounds. More than 5o0 exist in images of one another. nature.
→ Life is virtually exclusive in its use of one chirality or the other Of these ~500, the same 23 are used in Left: Amino acids all life on Earth.
Right: Nucleotides, Sugars Protocells and the RNA world
Laboratory experiments demonstrate that the ingredients for life could form in the conditions present on the early Earth. The formation of the membrane was likely a critical step in the development of life.
Membranes:
● Constrain important chemicals so that reactions happen more quickly ● Ensure the products of these reactions do not disperse ● Reduce the likelihood of a reaction with compounds from the outside world
With the formation of a membrane, the protocell could form. Within the protocell, RNA provided a fragile mechanism for information storage.
Once the protocell forms, natural selection guides the development of life. The mechanism of variation
Darwin, Wallace, and other pioneers of natural selection could not explain how variation was passed from parent to child...
We now know that that information is carried by a substance known as Deoxyribonucleic acid (DNA) How DNA works
● Regions of DNA that encode information about the construction of an organism are called genes. ● Genes can be copied into RNA, which serves as a template for proteins, a fundamental part of the cell. ● During DNA replication, mistakes, called mutations, can occur. ● If the organism lives long enough to reproduce, these mutations can be passed on to the next generation. A simple example
A population Members with a Less frequent Eventually that has a variable certain reproduction trait disappears trait expression of means fewer that trait survive expressions of less frequently that trait Natural selection in action From one to many
Eventually, the accumulation of traits means that two populations that started the same have become too different to reproduce with one another. This is known as speciation.
Different species maintain much of the same DNA.
Humans share 7% of their DNA with bacteria. This is the machinery of life. A slow, but powerful force
Modern primates began to diverge about 8 million years ago. When things go terribly wrong
90-96% of 75% of species die species die
60-70% of 70-75% of species die species die
Millions of years ago Rapid changes in the environment lead to mass extinction. The question of complexity
An active increase in complexity occurs when simple organisms always evolve into more complex ones.
→ Over time, most organisms become complex
A passive increase in complexity occurs when organisms evolve along a random walk.
→ Over time, few organisms become complex The question of complexity
So which process is at play?
→ No conclusive answer
SImpler organisms are more difficult to discover, making species numbers difficult to estimate.
→ Number of bacteria species estimated between tens of thousands and billions! The tree of life Finding the root of the tree
Because all life on Earth appears to function on a cellular level using the same basic machinery of life, we believe today that all living things can trace their evolution back to a single species, the last universal ancestor (LUA).
LUA Small group activity
The search for life elsewhere in the Universe is perhaps the ultimate scientific undertaking. But, how will we know when we’ve found it?
In a small group, discuss the following:
1) What specific aspects of possible extraterrestrial life could distinguish it from life here on Earth? 2) Is life elsewhere in the Solar System likely to be more similar to Earth life than life elsewhere in the MIlky Way or is all life likely to be unique? What evidence supports your postulate?
Be prepared to share your group’s answers with the class!