Diversity of life on Earth

Goals• Learn our working definition of life;• Evolution: processes and mechanisms;• Carbon-based life; silicon-based life;• Three domains of life.

The attributeRandom conditions are not a characteristic of life: order is required.

Exception #1Some forms of life do not look very ordered!

Exception #2But…the presence of order is not exclusive to therealm of life. It is a necessary, but not sufficient, condition. Abiotic order can occur on vast scales.

Six characteristics of life: #1—order

The attributeOrganisms on Earth use sexual or asexual processes to propagate themselves.

Asexual clones

Sexual genetic changes

Exception #1There are cases of life in which reproduction is impossible.

Exception #2Reproduction is a necessary but not sufficient requirement for life. Abiotic reproduction can occur.

Six characteristics of life: #2—reproduction

The AttributeEnlargement from a smaller state is necessary. Furthermore, a method of passing traits (characteristics defining the life form) must ensure the subsequent generations are (more or less) identical (i.e, heredity).

Exception #1Some organisms are incapable of growth to a larger state, and even get smaller with each generation.

Exception #2 Not everything that grows is alive.

Six characteristics of life: # 3—growth and development

The AttributeAll things tend to become disordered with time. This law of increasing entropy is basic to the Universe. To maintain order and to carry out daily activity, life must have an energy source.

Exception #1Life forms can enter dormant states where energy consumption drops to almost zero.

Exception #2 Nonliving processes can use an energy source to counter entropy, and create order.

Six characteristics of life: # 4—energy use

The AttributeOrganisms respond to changes in their local conditions. Organisms capable of movement move to adapt to changes in environmental conditions.

Exception #1Changes in the environment do not demand a response from the organism if the organism does not need to change its behavior.

Exception #2 Non-living physical processes or chemical reactions can be sensitive to the conditions, and respond accordingly.

Six characteristics of life: # 5—response to environment

The AttributeLife forms change over time in response to their environment. This response (adaptation) is usually slow, but results in enormous changes over long time periods.

Exception #1Some organisms are famous for not evolving—yet they do.

Exception #2 Complex nonliving systems can act like life, and evolve much as living systems.

Six characteristics of life: # 6—evolutionary adaptation

Almost-life

PrionsMost commonly believed to be nothing more than a misfolded protein.

Prions do not reproduce, but they induce other proteins to fold into the prion form.

Prions have the ability to affect neural tissue. Prions may attack by somehow aggregating in affected tissues.

Diseases include “mad cow disease” in cattle, chronic wasting disease in elk and deer, and Creutzfeldt-Jakob disease in humans. All prion diseases are currently fatal.

The word prion is derived from “proteinaceous” and “infectious.”

Almost-life

Virus (pl. viri, viruses)Although viruses fulfill most of the criteria of life, they are unable to reproduce without a host cell of another organism.

Viruses are not cellular.

They are not classifiable in the three domains of life (to be explained).

WHAT ARE THEY?

They are apparently only highly organized, protein-based, non-living structures, probably a byproduct of evolution.

Evolution: the concept

When organisms reproduce from one generation to the next, sexually or asexually, the descendent organisms are nearly identical to the parent organisms. Cows do not give birth to lions.

Due to genetic variations, the descendent organism can be slightly different from the parents. The hair may be slightly thicker; a bone might be slightly longer, etc.

Over time, a population of interbreeding organisms can change in character.

Three mechanisms for evolution: #1—survival pressures

If the individuals in a subsequent generation of slightly modified organisms are better adapted to the environment than other organisms, they are more likely to survive to reproduce.

This is called “natural selection”, as proposed by Charles Darwin (1859; On the Origin of the Species) and Alfred Wallace.

Artificial selection is what animal and plant breeders do, to develop new breeds.

In Darwin’s and Wallace’s time, DNA was unknown, so the mechanism of heredity was not understood.

Consider two identical but separated populations of organisms. Over time, random chance may result in some genetic characteristics disappearing in one population, but not in others.

The marble jar analogyConsider a container containing 50 white and 50 black marbles. Pull out a marble, and match that color with a new marble in a new jar.

Return the marble to the jar, shake it, and repeat 99 more times.

The new jar of marbles is unlikely to have 50 white and 50 black marbles. The population genetics have drifted.

Use this new jar of marble to create a third, etc. In time, the genetic drift will become even larger.

Three mechanisms for evolution: #2—genetic drift

Occasional catastrophes or opportunities may wipe out entire populations. Other events may open entire new vistas for new organisms.

Asteroidal impacts;

New island chains;

Dramatic climate changes;

Diseases wiping out species—catastrophe and opportunity!

Three mechanisms for evolution: #3—luck and opportunity

Darwin’s model

FactAny successful species, by the very nature of its success, will expand in population until its resources become depleted.

FactA population of organisms contain individuals with different characteristics, and some of these are inheritable.

Inescapable conclusionThose organisms with the best set of traits will survive better in the resource-depleted world. Their descendants will prosper and dominate.

Examples of evolution

Examples– Bacteria developing resistance to antibiotics;– Insects developing resistance to pesticides;– Plants developing resistance to herbicides;– Animals developing resistance to disease;– Selective breeding.

The tale of Biston betulariaThis peppered moth is normally overall white with black specks, although natural variation exists. The camouflage protects it from predators.

During the 1800s (industrial revolution), as British forests became covered with soot, the populations shifted to black moths.

With subsequent environmental reforms, populations of peppered moths shifted to the paler form.

Molecular support of evolution

Evolutionary thought, as promoted by Darwin, was much like Kepler’s laws.

It was a useful model; It successfully made accurate predictions; It was simple.

However, it lacked an underlying explanation. How did evolutionary changes from one generation get transmitted to the next?

The genetic revolutionThe knowledge of how genetics works, and how DNA maintains the library of knowledge needed to create an organism, provides a process explaining evolution, much as Newton’s laws provided a mathematical framework for Kepler’s laws.

Two misconceptions about evolution

MisconceptionEvolution evolves towards some future form.

Originating observation The fossil record shows that for the last 50 million years, horses have been evolving to a larger size.

Incorrect interpretation In the future, evolution will tend to make larger horses.

Evolutionary science says Evolution addresses the situation of the moment; it does not have momentum.

Two misconceptions about evolution

MisconceptionOrganisms can pass acquired attributes to subsequent generations.

Originating observation Giraffes have evolved long necks.

Incorrect interpretation Giraffes that stretch their necks into the trees to reach leaves get longer necks; such longer necks are passed to their offspring.

Evolutionary science says Necks stretched during the animal’s life is not transmitted genetically. Only hereditary characters can be passed to offspring.

Note: mice with cut-off tails do not give birth to half-tail offspring..

Creationist/ID charges against evolution

No intermediate fossils are found!This was once the case, but is no longer.

Thermodynamics is violated (entropy must increase)It is true that entropy always increases in a closed system. But the Earth is not a closed system. It is being heated by an external energy source (the Sun) and is radiating energy into space.

In the same way, you can decrease the entropy of an area, such as straightening up your room, but this takes work to do, thus increasing the entropy of the overall Universe.

Evolution does increase the entropy of the Universe, even though the local entropy of the Earth decreases. Evolution is powered by terrestrial and solar power sources, and waste energy is radiated into space in accordance with thermodynamics.

Eyes could not have evolved because they are too complex.Intermediate eyes, such as photosensitive surfaces, do have survival value. Evolution simply works to refine them by stages.

Cellular Life on Earth

Cell structure– All life is based on organic compounds (carbon-based);– All life forms on Earth are built upon cell structure;– Cell walls/membranes – inside vs. outside;– All life follows the same basic biochemistry, using only about 20

of the 70+ known amino acids;– All life on Earth is “left-handed” on a chemical level.

All life appears to have a common origin

Shadow biospheresAs the Earth remains largely unexplored, we cannot rule out additional life biochemistries. The discovery of a shadow biosphere would suggest either the independent development of life, or relic life from much earlier times.

Components of cells Carbohydrates

Molecules consisting of C, H, O, that serve as energy sources and structural components.

LipidsFat molecules. An essential component of membranes; can spontaneously produce chemical-trapping membranes where complicated chemistry could safely occur.

ProteinsComplicated molecules, built from amino acids, that can act as structural elements, enzymes (that dissolve other proteins), or catalysts that allow other chemical reactions to proceed rapidly.

Nucleic acidsRNA (ribonucleic acid), DNA (deoxyribonucleic acid).

Silicon-based lifeCarbon-based life

Because carbon can form four bonds, it can make complex molecules, such as are needed for biochemistry.

Silicon-based lifeCould you base life on silicon? It is just below carbon on the periodic table, and can make four bonds.

Sadly….Silicon cannot readily form double/triple bonds;

Some silicon molecules are unstable in water;

Silicon is rare in our universe (C:Si ratio is 10:1);

The Earth is silicon rich, but life is carbon-based.

Boron-based life, etc.Other possibilities exist. But all appear to have critical flaws. Boron, for example, is unfortunately very rare in the Universe.

Life form classifications: Prokaryotes

Attributes1. Essentially always unicellular.2. Cells are simple, with DNA not inside a nucleus.3. Asexual in reproduction.

Bacteria domain.Includes many disease organisms.

Archaea domain.Originally found only in extreme environments;Distinct from bacteria in many key chemical characters.

Life form classifications: Eukaryotes

Attributes1. Multicellular or unicellular.

2. Cells include a nucleus that stores the DNA. Other such “organelles” include mitochondria and chloroplasts.

3. Asexual or sexual in reproduction.

Eukarya domainIncludes all plants, animals, fungi, protozoans.

Three domains of lifeArchaea, eukarya, bacteria

Are there more domains, or perhaps shadow biospheres?