Evolution:

Biology as a science made its move from an Arisotitlean stage to a Newtonian one with the development of the theory of evolution. Evolution is a change in the gene pool of a population over time. A gene is a hereditary unit (the microscopic `atom') that can be passed on unaltered for many generations. The gene pool is the set of all genes in a species or population (the macroscopic `object').

The English moth, Biston betularia, is a frequently cited example of observed evolution. In this moth there are two color morphs, light and dark (typica and carbonaria). H. Kettlewell found that dark moths constituted less than 2% of the population prior to 1848. Then, the frequency of the dark morph began to increase. By 1898, the 95% of the moths in Manchester and other highly industrialized areas were of the dark type, their frequency was less in rural areas. The moth population changed from mostly light colored moths to mostly dark colored moths. The moths' color was primarily determined by a single gene. So, the change in frequency of dark colored moths represented a change in the gene pool. This change was, by definition, evolution.

The increase in relative abundance of the dark type was due to natural selection. The late eighteen hundreds was the time of England's industrial revolution. Soot from factories darkened the birch trees the moths landed on. Against a sooty background, birds could see the lighter colored moths better and ate more of them. As a result, more dark moths survived until reproductive age and left offspring. The greater number of offspring left by dark moths is what caused their increase in frequency. This is an example of natural selection.

Populations evolve, not individuals. In order to understand evolution, it is necessary to view populations as a collection of individuals, each harboring a different set of traits. A single organism is never typical of an entire population unless there is no variation within that population. Individual organisms do not evolve, they retain the same genes throughout their life. When a population is evolving, the ratio of different genetic types is changing -- each individual organism within a population does not change. For example, in the previous example, the frequency of black moths increased; the moths did not turn from light to gray to dark in concert.

The process of evolution can be summarized in three sentences: Genes mutate. Individuals are selected. Populations evolve.

Thomas Malthus (1766-1834) was an English clergyman, whose writings on population growth had a strong influence on the theory of evolution by natural selection developed by Charles Darwin and Alfred Russel Wallace.

In An Essay on the Principle of Population (1797), Malthus observed that most organisms produce far more offspring than can possibly survive.

Even when resources are plentiful, the size of a population tends to increase geometrically until the population outstrips its food supply. This led Malthus to believe that poverty, disease, and famine was a natural and inevitable phenomenon, leading to a "struggle for existence".

Evolution came of age as a science when Charles Darwin published "On the Origin of Species." Darwin's contributions include hypothesizing the pattern of common descent and proposing a mechanism for evolution -- natural selection.

Darwin read Lyell's Principles of Geology and came to accept Lyell's view that long-term geological processes were responsible for shaping the earth's surface in a gradual manner. Indeed, Darwin successfully applied uniformatarianism to explain the development of coral reefs.

In Darwin's theory of natural selection, new variants arise continually within populations. A small percentage of these variants cause their bearers to produce more offspring than others. These variants thrive and supplant their less productive competitors. The effect of numerous instances of selection would lead to a species being modified over time.

Selection:

Some types of organisms within a population leave more offspring than others. Over time, the frequency of the more prolific type will increase. The difference in reproductive capability is called natural selection. Natural selection is the only mechanism of adaptive evolution; it is defined as reproductive success of classes of genetic variants in the gene pool.

Natural selection can be broken down into many components, of which survival is only one. Sexual attractiveness is a very important component of selection, so much so that biologists use the term sexual selection when they talk about this subset of natural selection. Sexual selection is natural selection operating on factors that contribute to an organism's mating success.

Three examples of selection are shown before stabilizing, disruptive and directional. The black dots are individuals that die out before passing on their genes. Stabilizing removes the extremes end of a trait distribution. An example might be birth weight of humans. Mortality rates at birth are highest at both ends of the normally distributed birth size range curve, thus tending to keep birth weight constant and near the mean. Directional selection would occur if individuals at one end of the normally distributed curve are favored. Disruptive selection would occur if selection simultaneously favored individuals at both ends of the curve, resulting in a tendency for the curve to become bimodal. An example is exhibited by butterflies, in which the females exist in several morphs some of which resemble two other species which are noxious. Intermediate butterflies do not gain the advantage of mimicry and thus are more likely to be preyed upon.

A new species diverges from its parent species as a small isolated population. According to the gradualist model, species descended from a common ancestor diverge more and more in morphology as they acquire unique adaptations. According to proponents of the punctuated equilibrium model, a new species changes most as it buds from the parents' lineage and then changes little for the rest of its existence.

Human Evolution:

Most of human evolution involves physical evolution, cultural evolution plays a fairly minor role until the Upper Paleolithic, 40,000 years ago. Proto-humans, hominids, were constrained and directed by the same evolutionary pressures as the other organisms they shared the ecosystem with.

Around 13 million years ago, a tree-dwelling primate developed:

  1. steroscopic vision
  2. high mobility (upright stance)
  3. opposable thumbs
Upright walking was a response to environmental changes in East Africa at the time, the rainforest was turning into steppes due to global weather changes. An upright stance is a survival characteristic to see over tall grasses. About 3.5 million years ago, our first direct ancestor appeared, Australopithecus africanus, whose best fossil example is should below.

This primate eventually evolved into Homo Sapian. Note, that IQ was not an early trait of hominids. Brain capacity increased to process more complicated visual information and due to increased physical body size. A side benefit from increased brain size was 1) profit from experience (memory/learning) and 2) the ability to choose between alternatives (reasoning). Both of these new capabilities lead to the skills needed to manipulate the environment (tools).

This illustration compares the crania of a female gorilla, Australopithecus africanus, and Homo sapiens. The dark area at the bottom of the skull is the foramen magnum, the hole through which the spinal column passes. It has a forward position in australopithecine skulls, a strong indication that they were bipedal. Note also that both the shape of the jaw and the teeth of australopithecines are very similar to those of modern humans. Australopithecines do not have the rectangular-shaped jaw or the large canine teeth of apes.

The idea that man evolved a large brain first was propagated for most of the 20th century by the famous Piltdown Hoax. When, in fact, most of the physical attributes of human form (upright walking, jaw and teeth structure, pelvic and leg formation) came before brain size evolved.

Our current idea of the human family tree is shown below, whose origins lie on the continent of Africa, then spread around the globe. We also know that every living human is the direct descendent of a single Homo Sapian woman who lived in Africa 150,000 years ago (i.e. Eve) based on the matching of DNA from cellular mitochondria in people around the world. Notice that our last common ancestor with apes is Australopithecus ramidus, about 5 million years ago. Also note that many species of Australopithecus and Homo are now extinct.

Which came last?

At the point where early Homo Sapian developed language a new form of evolution began. Normal evolution has inherited traits being transmitted by genes. So a bird knows how to build a nest due to inherited learning. However, language now allows the passing on of information by behavioral means, the process of learning and teaching. Although we humans are genetically equipped with basic biological imperatives, our sophisticated cultural behavior must be learned and language is the symbolic mode of communication that is associated with this learning.

The basic premise here is that culture has some advantage for the survival of our ancestors, therefore natural selection favors genes responsible for such behavior. DNA information only passes from individual to individual, but cultural evolution is active, incorporates a lifetime of teaching and can be passed from one individual to many. Cultural evolution, with its global nature, becomes the distinguishing characteristic of humans.

Is there Life Out There?

Perhaps the most important discovery humankind could ever make would be the discovery of life outside the Earth.

The search for life outside the Earth actually starts on the Earth with the investigation of meteors. Carbonaceous chondrites have been found to contain organic molecules, proteins and amino acids. Interestingly, there are equal numbers of left-handed and right-handed amino acids in meteors, whereas on the Earth all amino acids are left-handed. On the Earth this is due to the fact that chemical evolution eliminated all right-handed macromolecules. Thus, amino acids in meteors must represent samples from the early stages of the Solar System before chemical evolution.

Over 800 pounds of lunar soil was returned by the Apollo missions. All of it was tested for organic materials. The only carbon found was in carbide, CH4 or CO, no amino acids or proteins. The bombardment of the lunar surface by high energy particles probably prevents the formation of macromolecules, and breaks down the ones from earlier times.

The Viking mission placed two landers on Mars, each containing three experiments to search for life:

  1. Pyrolytic release - an experiment to test for photosynthesis, where a small amount of martian soil was placed in a CO2 gas, using carbon-14, illuminated for a time, then baked. If living organisms ingest the CO2, then the soil would contain traces of the isotope.
  2. Label release - an experiment to look for metabolism, where a small amount of martian soil is moistened with nutrients tagged with carbon-14. If living organisms exist they would release the carbon-14 as waste.
  3. Gas exchange - an experiment to test for respiration, where a sample of soil is given nutrients in a controlled atmosphere. The atmosphere is monitored for changes.
The first two experiments showed rapid changes in the martian soil, but too fast for most living processes. The martian soil is rich in oxides, and the reactions seen where chemical in nature.

Life in Other Solar Systems:

There is strong evidence for planets around other stars. But, for life to develop several requirements must be met:

  1. heavy elements - for life to develop there must have been a nearby supernova explosion to provide sufficient elements heavier than helium
  2. bioshell - the planet must be at the proper distance from the star for liquid water
  3. chemical evolution - there must be a chemical build-up and evolutionary process
  4. energy source - there must be a local energy source for life processes

Non-Carbon Based Life

All lifeforms on this planet are based on hydrocarbons, i.e. the carbon atom is the atomic ``glue'' in our bodies. Thus, all assumptions we make about life on other planets is based on the properties of carbon. Is it possible all our theories are wrong and that non-carbon based life is possible?

The next element of the periodic table with the same atomic properties as carbon is silicon (Si). Can hydro-silicon compounds be formed as a starting point for silicon based life forms? The answer is that hydro-silicon molecules are possible, but since the silicon atom is much heavier than the carbon atom, the atomic bonds for silicon are only stable at much lower temperatures than for carbon (on the order of -150 F). So hydro-silicon compounds might form, for example, on the moons of the Jovian worlds where the temperatures are low.

Unfortunately, low temperature also mean low chemical reaction rates. One of the requirements for complex lifeforms is a high rate of chemical reactions to supply the energy for mechanical action. It is possible to imagine primitive silicon based lifeforms, but they would not be able to compete with higher speed carbon-based lifeforms.