Origin of Life:

The Earth's crust became stable about 3.9 billion years ago. Life appeared around 3.6 to 3.9 billion years ago, which is quite fast in astronomical terms. Microfossils found in ancient rocks from Australia and South Africa demonstrate that terrestrial life flourished by 3.5 billion years ago. Older rocks from Greenland, 3.9 billion years old, contain isotopic carbon, carbon that could only have belonged to a living organism. The early atmosphere of the Earth was a secondary atmosphere from volcanic outgassing, very CO2-rich with little free O2.

The Earth lies at the correct distance from the Sun for liquid water to exist. The evolution of life requires two elements; energy and a medium for growth. Sunlight serves as the source of energy for most life (a counter-example is bacteria that grows on the ocean trenches powered by heat from thermal vents). Sunlight provides the energy needed for food manufacture (biochemical energy storage) and molecular construction (genetic material, cell walls, etc.). Indirectly, sunlight provides a warm temperature, which means higher chemical reaction rates for simple life. More complex life requires sunlight for vision and a stable environment.

Chemical Evolution:

Liquid water provides a universal solvent and warm environment for chemical evolution. It is a vehicle for dissolved substances (it circulates). And it provides the raw material for protein construction.

When the primordial soup is exposed to energy, organic compounds are produced as shown by the Miller-Urey Experiment.

Amino acids are small, highly reactive molecules composed of 20 to 30 HCNO atoms. When amino acids link together in strings they form proteins. Proteins govern chemical reaction rates and form the structural material for cell parts.

Most importantly, they can form into microspheres when heated, which serves to separate chemical reactions and processes. The problem is that with the vastness of the Earth's oceans it is statistically very improbable that these early proteins would ever link up. The solution is that the huge tides from the Moon produced inland tidal pools, which would fill and evaporate on a regular basis to produce high concentrations of amino acids, who then linked themselves into macromolecules.

With the construction of large macromolecules, such as proteins and nucleic acids, the Earth is poised for the next stage of biochemical evolution. Living organisms are the supreme example of active matter. They represent the most developed form of organized matter and energy that we know. They exemplify growth, adaptation, complexity, unfolding form variety and unpredictability. Almost appearing to be a class apart from matter and energy, defying the laws that enslave normal matter and energy.

Every organism is unique, both in form and development. Unlike physics where one studies classes of identical objects (e.g. electrons, photons), organisms are all individuals. Moreover, collections of organisms are unique, species are unique, the evolutionary history of the Earth is unique, the entire biosphere is unique. On the other hand, a cat is a cat, a cell is a cell, there are definite regularities and distinguishing features that permit organisms to be classified.

Each level of biology has new and unexpected qualities, qualities which cannot be reduced to the properties of the component parts, this is known as holism. A living organism consists of a large range of components differing greatly in structure and function (heart, liver, hair). Yet, the components are arranged and behave in a coherent and cooperative fashion as though to a common agreed plan. This endows the organism with a discrete identity, makes a worm a worm, a dog a dog.

No living thing exits in isolation. All organisms are strongly coupled to their inanimate environment and require a continual throughput of matter and energy as well as the ability to export entropy. From a physical and chemical point of view, every organism is strongly out of equilibrium with its environment. In addition, life on Earth is an intricate network of mutually interdependent organisms held in a state of dynamic balance. Then concept of life is fully meaningful only in the context of the entire biosphere.

A large number of complex chemical reactions is the underlying process that we call life. The ingredients for life are:

  1. energy source
  2. supply of nutrients (building blocks)
  3. self-regulating mechanisms
The first two criteria were supplied by the conditions of the early Earth environment. The third criteria was presented by the endpoint of chemical evolution where the long chains of nucleic acids were formed which developed into RNA and DNA.

RNA and DNA are molecular codes for the production of proteins. They have the unique property of being self-replicating (when an RNA molecule splits, amino acids connect to the endpoints producing an exact copy of the original chain). The beginning of biochemical evolution was when RNA and DNA evolved to coat themselves in protein shells. These coated RNA and DNA packages are called a virus. A virus is halfway between life and non-life, being non-living when in isolation, but adapting living characteristics in interaction with other virus' or cells.

The next stage in biochemical evolution was for various virus' to take on specialized tasks (energy production, protein production, etc). These individual elements would combine to form the first cell. Our earliest evidence of cellular life comes from fossil bacteria.

With the development of cells, life took on an explosive evolution into more diverse forms, invading new environments (sea, lakes, land).

Photosynthesis:

Oxygen is a very small component to outgassing on the Earth, yet O2 is a significant fraction of our current atmosphere (thank goodness). Also note that O2 is highly reactive and combines quickly with rock and soil to form oxides (rust). Thus, the current amount O2 requires a constant process of replenishment. That process is photosynthesis.

The first photosynthesizing organisms used UV light as an energy source since there is more energy associated with short wavelength light than long wavelength light (want proof? leave your shirt off for an hour at the beach). This occurred about 3.5 billion years ago and the immediate by-product was the ozone layer, which blocks UV light. This resulted in the first mass extinction, the death of all UV photosynthesizing cells. Only organisms which were able to utilize the visible portion of the spectrum survived = green plants and plankton.

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.

Fermi's Paradox (i.e. Where are They?):

The story goes that, one day back on the 1940's, a group of atomic scientists, including the famous Enrico Fermi, were sitting around talking, when the subject turned to extraterrestrial life. Fermi is supposed to have then asked, "So? Where is everybody?" What he meant was: If there are all these billions of planets in the universe that are capable of supporting life, and millions of intelligent species out there, then how come none has visited earth? This has come to be known as The Fermi Paradox.

Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire Galaxy. Within a few million years, every star system could be brought under the wing of empire. A few million years may sound long, but in fact it's quite short compared with the age of the Galaxy, which is roughly ten thousand million years. Colonization of the Milky Way should be a quick exercise.

So what Fermi immediately realized was that the aliens have had more than enough time to pepper the Galaxy with their presence. But looking around, he didn't see any clear indication that they're out and about. This prompted Fermi to ask what was (to him) an obvious question: "where is everybody?"

While interstellar distances are vast, perhaps to vast to be conquered by living creatures with finite lifetimes, it should be possible for an advanced civilization to construct self-reproducing, autonomous robots to colonize the Galaxy. The idea of self-reproducing automaton was proposed by mathematician John von Neumann in the 1950's. The idea is that a device could 1) perform tasks in the real world and 2) make copies of itself (like bacteria). The fastest, and cheapest, way to explore and learn about the Galaxy is to construct Bracewell-von Neumann probes. A Bracewell-von Neumann probe is simply a payload that is a self-reproducing automaton with an intelligent program (AI) and plans to build more of itself.

Attached to a basic propulsion system, such as a Bussard RamJet (shown above), such a probe could travel between the stars at a very slow pace. When it reaches a target system, it finds suitable material (like asteroids) and makes copies of itself. Growth of the number of probes would occur exponentially and the Galaxy could be explored in 4 million years. While this time span seems long compared to the age of human civilization, remember the Galaxy is over 10 billion years old and any past extraterrestrial civilization could have explored the Galaxy 250 times over.

Thus, the question arises, if it so easy to build Bracewell-Von Neumann probes, and they has been so much time in the past, where are the aliens or at least evidence of their past explorations (old probes). So Fermi Paradox becomes not only where are They, but why can we not hear Them and where are their Bracewell-von Neumann probes?

If one considers the amount of time the Galaxy has been around (over 10 billion years) and the speed of technological advancement in our own culture, then a more relevant point is where are all the super-advanced alien civilizations. Russian astrophysicist Nikolai Kardashev proposed a useful scheme to classify advanced civilizations, he argues that ET would posses one of three levels of technology. A Type I civilization is similar to our own, one that uses the energy resources of a planet. A Type II civilization would use the energy resources of a star, such as a Dyson sphere. A Type III civilization would employ the energy resources of an entire galaxy. A Type III civilization would be easy to detect, even at vast distances.

This sounds a bit silly at first. The fact that aliens don't seem to be walking our planet apparently implies that there are no extraterrestrial anywhere among the vast tracts of the Galaxy. Many researchers consider this to be a radical conclusion to draw from such a simple observation. Surely there is a straightforward explanation for what has become known as the Fermi Paradox. There must be some way to account for our apparent loneliness in a galaxy that we assume is filled with other clever beings.

Possible solutions to Fermi's Paradox fall in the following categories:

In general, solutions to Fermi's paradox come down to either 1) life is difficult to start and evolve (either hard for the process or hard to find the right conditions) or 2) advanced civilzations destroy themselves on short timescales. In other words, this is an important problem to solve in the hope that it is 1 and not 2.