Where did the different types of organisms now living on earth come from?  This is one of the fundamental questions that biology attempts to answer. 

Did organisms arise at one specific time point, or have different types of organisms appeared at different times during the history

It used to be widely believed [link] that various types of organisms, such as flies, frogs, and even mice, arose spontaneously, from non-living matter.

Flies, for example, were thought to appear from rotting flesh and mice from wheat. If true, on-going spontaneous generation would have profound implications for our understanding of biological systems.

A key event in the conceptual development of modern biology was the publication in 1668 of Francesco Redi's "Experiments on the generation of insects". He hypothesized that spontaneous generation did not occur and that the organisms that appeared had developed from "seeds" deposited by adults.

His hypothesis predicted that if adult flies were kept away from rotting meat, for example, maggots would not appear - no matter how long one waited. 

To test this hypothesis, 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 could arise only from other insects.

The invention of the microscope, and its application to biological materials by Antony van Leeuwenhoek (1632-1723) and Robert Hooke (1635-1703), led to the discovery of a completely new (and totally unexpected) world of microscopic organisms - the protozoa, microscopic fungi, bacteria, and archaea.

A number of scientists began to explore the reproduction of microbes. Lazzaro Spallazani (1729-1799) showed that after a broth was boiled, it remained sterile (that is, without life) as long as it was isolated from contact with fresh air.

He concluded that microbes, like larger organisms, could not arise spontaneously, but were descended from other microbes (a version of the cell theory).

One criticism of this experiment was that perhaps the process of boiling the broth destroyed some vital component necessary for the formation of microbes, or perhaps fresh air was the "vital" ingredient.

In 1862, Louis Pasteur carried out a particularly convincing set of experiments to address these concerns. He sterilized broths by boiling them 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 for months. Once the neck of the flask was broken, however, the broth was quickly overrun with microbial growth. 

Based on such experiments, a consensus was reached that neither microscopic nor macroscopic organisms could arise spontaneously, at least in the modern world.

Because of this scientific consensus, studying spontaneous generation was no longer considered a productive way to advance one's career in science.

1. Why did the discovery of bacteria reopen the debate on spontaneous generation?
2. What were the positive and negative controls in Pasteur's experiment?

• How would biology change if  spontaneous generation were occurring today, for example, between your toes?

Where did the first organisms come from? To begin to address this question, it is best to start at the beginning.  The current scientific model for the origin of the universe is known as the "Big Bang" theory. 

Edwin Hubble (1889-1953) was the first to realize that the fuzzy nebula that astronomers saw through their telescopes were in fact galaxies, like our own Milky Way.  Each was composed of many millions of stars.

These galaxies appeared to be moving away from one another. He concluded that at one point in the past, all of the matter and energy in the universe must have been concentrated in a single point.

Based on this hypothesis, it is possible to estimate the age of the universe at 13.7 +/- 0.2 billion (10⁹) years. This is a length of time well beyond human comprehension; it is sometimes referred to as deep time.

The earth and the other planets formed ~ 4.5 x 10⁹ years ago. The earliest period of earth history is known as the Hadean, after Hades, the Greek god of the dead. The Hadean is defined as the period before the appearance of life. The first evidence of biological processes appear in rocks that are ~3.8 to 3.5 x 10⁹ years.

We do not know how many times life arose on the early earth, all we do know is that all of the current organisms we have studied are descended from a single common ancestor.

Evidence of life: Fossils provide the most dramatic evidence of the history of life on earth. Fossils are formed only in sedimentary rock. There are a

Chemical fossils are molecules that, as far as we know, occur in naturally only through biological processes. Their presence in ancient rock implies that living organisms were present at the time the rock was formed. What makes chemical fossils problematic is that there may be non-biological mechanisms that could produce them, but which we have yet to discover.

Trace fossils can be subtle or obvious.  Burrowing animals can leave tunnels and disrupt layers of sediment.  Animals that walk can leave footprints.

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

Dinotracks at Dinosaur Ridge, in Morrison, Colorado

Structural fossils are the mineralized remains of organisms. More often than not a dead organism is eaten, and nothing recognizable remains.

Occasionally an organism's remains are buried before they are destroyed. This can lead to fossilization, which involves replacing the organic materials with minerals. Over the billions of years that life has existed on earth, many fossils have been created; as erosion removes surface layers of rock, these fossils come to light.

A fossil can be as simple as a single tooth, scale or shell, or as complex as a complete skeleton. As organisms developed hard parts, such as bones, teeth, and shells, fossilization became more likely. 

Fossils of various types suggest that the first living microbes were present on earth by ~3.5 x 10⁹ years ago, and perhaps earlier. 

Change over time: Early on in the history of geology, it was discovered that fossils of specific types were found associated with rocks of specific ages - this correlation was so robust, that rocks can be accurately dated by the types of fossils they contain.

The oldest rocks contain the fossilized remains of only very simple organisms.

Life's impacts: 

Based on fossil evidence, it would appear that for a period of ~2 x 10⁹ years, microscopic organisms were the only form of life on earth.

During this period, photosynthetic bacteria captured light and used that energy to drive the reaction by which CO₂ (carbon dioxide) and H₂O (water) are combined to form sugars (carbohydrates).

In the course of these reactions molecular oxygen (O₂) was produced as a waste product. Over time O₂ began to accumulate in the atmosphere. By about 300 million years ago, atmospheric O₂ levels had reached ~35%, almost twice the current level. 

Some hypothesize that it was these high levels of atmospheric O₂ that made possible the evolution of giant flying insects. 

Because O₂ is a highly reactive and potentially toxic compound, its appearance posed challenges and provided opportunities to many organisms. 

For example, through coupled reactions O₂ can be used to extract the maximum amount of energy from food, leading to the generation of CO₂ and H₂O.

Around 10⁹ years ago trace fossil burrows appeared; these were likely to have been produced by simple worm-like, macroscopic (meaning larger) metazoans (meaning multicellular animals) moving along and through the mud on the ocean floor. 

About 600 x 10⁶ years ago, new, more complex structural fossils begin to appear in the fossil record. The first of these were the Ediacaran organisms. It remains unclear how they are related to later organisms. 

By the beginning of the Cambrian age (545 x 10⁶ years ago), a wide variety of organisms had appeared, many clearly related to modern organisms. 

These Cambrian organisms show a range of body types.  Most significantly, many were armored, suggesting the value of adaptations against predators. 

Since the fossil record does not contain all types of organisms, we are left to speculate on what the earliest metazoans looked like.

1. How do scientists estimate the age of the universe?
2. What factors would influence the probability that a particular organism, or type of organism, would be fossilized? 
3. What factors might have driven the appearance of teeth, bones, and shells?
4. How would biology change if fossils of complex organisms (such as vertebrates) were to be found in the most ancient of rocks?  
5. Propose a model for why O₂ levels have changed so dramatically over the course of earth's history.

• If life first arose ~3.5 x 10⁹ years ago, why did it take so long for complex multicellular organisms to form?
• What would evidence for pathogens (disease-producing organisms) look like in the fossil record?

Understanding origins: There are at least three possible approaches to the study of life's origins. A religious (i.e. non-scientific) approach postulates that life was created by a supernatural being or process.

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

Nevertheless, "intelligent design" creationists claim that we can identify those aspects of life that could not have been produced by natural processes, by which they mean the processes of evolution. This approach abandons science, and displays a failure of faith in the creative power of natural processes, as well as in our ability to discover and understand them. 

More significantly, it implies that the origins of life are, by definition, beyond the ability of science to study. The lesson of history, however, is different. Predictions as to what is "beyond the ability of science to explain" have consistently been demonstrated to be wrong, often only a few years after they are made!

Another type of explanation would be to assume that advanced aliens brought life to earth. This hypothesis is termed panspermia. Perhaps we owe our origins to casually discarded litter from alien visitors.  

This view was challenged when Friedrich Wöhler synthesized urea in the lab in 1828. 

Urea is a simple organic molecule. O=C(NH₂)₂. It is naturally found only in living organisms and is a major waste product.  Urine contains lots of urea. 

Wöhler's in vitro or "in glass" synthesis of urea was simple. He took the inorganic compound ammonium cyanate (NH_4⁺CNO^-) and heated it; this led to the production of urea. 

One approach to understanding what is involved in the origin of life is to attempt to create the precursors of living systems in the laboratory. An early and influential 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 experimental atmosphere.  

After awhile (hours/days), they found that a complex mix of compounds had formed. Included in this mix were many different types of amino acids.

Certain types of meteorites also contain complex organic molecules. It therefore appears likely that the early earth was rich in organic (that is, carbon-containing) molecules, the building blocks of life.

Given that the potential building blocks were present, the question is, what set of steps were involved in generating living systems? The earliest proto-biotic systems were likely to be molecular communities of chemical reactions isolated in some way from the rest of the "outside" world. 

One possible model is that such systems were originally associated with the surface of specific minerals, which served as catalysts for important reactions.

Over time, these systems acquired membranes and were able to exist free of the mineral surface. 

Such an isolated system has important properties that are likely to have facilitated the further development of life.  

For example, because of the membrane boundary, changes that occur within one such structure will not be "shared" with neighboring systems, they can accumulate and favor the "survival" of one system over its neighbors. 

Such systems can also "divide" in a crude way by fragmentation.  If changes within one such system improves its stability, its ability to accumulate resources, or its ability to survive and reproduce, that system, and its progeny, are likely to become more common.

Scientists now believe they may be able to make very simple 'artificial cells' which can metabolize, replicate and evolve. - Jack Szostak

1. What are the properties of life?
2. What did Wohler's synthesis of urea and the Miller/Urey experiment actually prove, versus what did they imply?
3. How did Wohler's experiment change the meaning of the term "organic"?  
4. If thermodynamics tells us that systems become more disordered over time, does life speed up or slow down this process?
5. Does life obey or disobey the second law of thermodynamics?

• What would it take for you to consider a computer to be alive?
• How is the idea of  "vitalism" similar to "intelligent design creationism"?  How is it different?