A key moment in the development of modern biology occurred in 1668 with the publication of Francesco Redi's Experiments on the generation of insects.

At that time it was widely believed that flies, frogs and mice could arise spontaneously.

Flies, for example, were thought to appear in rotting flesh.

The alternative was they developed from "seeds" deposited by adults.

To test whether the appearance of maggots depended upon adult flies Redi set up two sets of flasks - both contained meat.

One set was exposed directly to the air (and so to flies), the other was sealed with paper or cloth.

Maggots appeared only in the flasks open to the air. Redi concluded that organisms as complex as insects arose only from other insects.

The invention of the microscope lead to the discovery of new and unexpected worlds of microscopic organisms - the protozoa, microscopic fungi and bacteria.

Antony van Leeuwenhoek, who lived from 1632 to 1723 and Robert Hooke who lived from 1635 to 1703, pioneered the exploration of this new world.

A number of scientists began to explore the reproduction of microorganisms.

Lazzaro Spallazani, who lived from 1729 to 1799, showed that a broth could be sterilized by heat. If the broth was isolated from the air, no microorganisms appeared.

He concluded that microorganisms, like larger organisms, could not arise spontaneously, but were descended from other microorganisms.

Some criticized this experiment by arguing that perhaps boiling destroyed a vital component in the broth necessary for the spontaneous formation of microorganisms.

In 1862, Louis Pasteur carried out a particularly convincing experiment to address this possibility. He sterilized broths in special "swan-necked" flasks.

The flask was open to the air but because of the shape of its neck, airborne organisms could not reach the broth.

The liquid remained sterile, that is free of microorganisms, for months.

Once the neck of the flask was broken, however, the broth was quickly overrun with microbial growth.

Based on these and many other experiments, a consensus was reached that neither microscopic nor macroscopic organisms could arise spontaneously.

This implies that there is a continuity to life, an unbroken lineage from its first origins to all modern day organisms.

We can find evidence of the history of life on earth in fossils. There are a number of different types of fossils.

Chemical fossils are molecules, or the breakdown products of molecules, that appear to be synthesized only by biologic processes. That means we know of no way that they can be formed naturally in the absence of life.

Their presence in ancient rock implies that living organisms were present at the time the rock was formed.

Fossil microorganisms

Structural fossils are the mineralized remains of organisms.

They can be as simple as a single tooth or scale or as complex as a complete skeleton.

Trace fossils can be subtle. Burrowing animals can leave tunnels, animals that can walk can leave footprints.

Organisms without hard parts, such as jelly fish, can leave impressions, much like footprints. .

You can see dinotracks in person at
Dinosaur Ridge, in Morrison, Colorado.

Coprolites are the fossilized excrement of ancient organisms.

The current best estimate for the age of the universe is between 12 to 14 billion years or 10⁹ years. The scientific prefix for 10⁹ is giga as in gigabytes.

This is a length of time almost beyond human comprehension and is sometimes referred to as deep time.

The earth and the other planets formed ~4,500,000,000 years ago or 4.5 gigayears ago, abbreviated Gya.

The earliest period of earth history is known as the Hadean, after the Greek god of the dead Hades. The Hadean lasted from the formation of the earth 5 to 4.5 Gya until the formation of the oldest preserved rocks, ~3.8 Gya.

The Hadean is defined as the period before the appearance of life. The first evidence of biologic processes appears in rocks that are ~3.8 gigayears old.

The first unambiguous fossil microbes can be found in rocks from ~3.5 to 3 Gya.

These were presumably proceeded by simpler prebiotic systems.

The molecular nature of these prebiologic systems remains unclear.

In large part, our ignorance stems from the uniqueness and unity of life.

We know only about life on the earth, and life on earth seems to have had a single common ancestor.

Whether or not alternative 'living' systems are possible and how they might differ from life on earth, is unknown.

There are alt least three possible approaches to the study of life's origins.

A religious approach would postulate that life was created by a supernatural being or process.

Different religious tradition differ as to the details of this event, but since the process is supernatural it cannot by studied further scientifically.

A science fiction approach would be to travel to distant worlds and look for life. We might even 'get lucky' and have a visit from extraterrestrials.

Scientific study of these extraterrestrials would reveal whether all living systems are built in the same manner or whether there are many different ways to "be alive".

An alternative, more practical approach is to attempt to create living systems or their precursors in the laboratory.

A early example of this approach was the Miller-Urey experiment.

These two scientists made a guess as to the composition of earth's early atmosphere. They assumed the presence of oceans and lightning.

They set up an apparatus to mimic these conditions.

They passed electricity through their atmosphere. Within the complex mix of compounds synthesized, they found many different types of amino acids, the building blocks of proteins.

Urea is a simple 'organic' molecule.

Originally, it was thought that organic molecules like urea could be synthesized only by living organisms. That they represented the action of the organism's "vital essence".

Wöhler's in vitro or 'in glass' synthesis of urea indicated that there was nothing "superchemical" or "supernatural' about living things, they obeyed the laws of chemistry.

The earliest proto-biotic systems were almost certainly molecular communities of chemical reactions isolated one way or the other from the rest of the "outside" world.

Whether or not these systems originated on earth, as seems likely, hardly matters to the basic problem of understanding their origins.

These proto-biotic systems extracted energy from the environment and used it to maintain their structural organization -- they exported entropy.

Such systems have an important limitation - they are vulnerable to accidental destruction.

They are also static. A small change in their environment can destroy them.

The key to avoiding destruction is reproduction. Making copies reduces the chance that a system will be eliminated by accident. It also opens the possibility for evolution.