This is one of the fundamental questions biology attempts to answer.    Did organisms arise a one specific time point, or have different types of organisms appeared at different times during the history of the earth?

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

Flies, for example, were thought to appear from rotting flesh and mice from wheat. 

If true, spontaneous generation would have profound implications for our understanding of biological systems.

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

Redi's hypothesized that spontaneous generation did not occur.  He assumed that the organisms that appeared had developed from "seeds" deposited by adults.

This hypothesis predicts that if adult flies are kept away from rotting meat, for example, maggots will not appear - no matter how long you wait. 

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, and bacteria.

A number of scientists began to explore the reproduction of microbes.   In particular, Lazzaro Spallazani (1729-1799), showed that after a broth was boiled, it would remain 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.

One possible 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 experiment 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 consensus, studying spontaneous generation was no longer considered a smart way to advance one's career in biology. 

• What types of controls could you add to Redi's experiments?
• What is a control experiment?
• How would biology change if spontaneous generation were occurring today (say between your toes)?
• Are scientists close minded, not to continue to look for spontaneous generation in the modern world?

How much time do we have (to generate life)?

In the absence of spontaneous generation, how did life originate? Where do organisms come from? If organisms can arise only from pre-existing organisms, where and when did the first organisms appear?

To begin to answer these questions, it is best to start at the beginning. 

The current scientific model for the origin of the universe is known as the "Big Bang". 

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

These galaxies were moving away from one another.   He concluded that at one point in the past, all of the matter and energy in the universe had 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 (a billion years is a gigayear and Gya stands for gigayears ago). 

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: we use the symbol "~" to mean "approximately".

The earliest period of earth history is known as the Hadean, after Hades, the Greek god of the dead.  

The Hadean period lasted from the formation of the earth, ~4.5 x 10⁹ years ago, until the formation of the oldest preserved rocks, which are ~3.8 x 10⁹ years old.

The Hadean is also defined as the period before the appearance of life. The first evidence of biologic processes appear in rocks that are ~3.8 to 3.5 x 10⁹ years old.

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 number of different types of fossils.

Chemical fossils are molecules that, as far as we know, can be synthesized 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 them problematic is that there may be abiological mechanism that we have not yet discovered that could produce them.

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 jelly fish, can leave impressions, much like footprints.

Dinotracks at Dinosaur Ridge, in Morrison, Colorado

Structural fossils are the mineralized remains of organisms.

They can be as simple as a single tooth, scale or shell, or as complex as a complete skeleton.  Generally, as organisms developed hard parts (bones, shells) their fossilization became more likely; nevertheless, fossilization is a rare occurrence. 

More often the dead organism is digested, and nothing recognizable remains. The study of what happens to the bodies of organisms when they die is known as taphonomy.  

Nevertheless, 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.

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 only very simple organisms. 

Life's impact:  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 transform CO₂ (carbon dioxide) and H₂O (water) into sugars (carbohydrates).

During this reaction, molecular oxygen (O₂) is produced as a waste product.  

Over time, O₂ began to accumulate in the atmosphere.  By about 300 x 10⁶ years ago, atmospheric O₂ levels reached ~35%, almost twice the current level.   

Some hypothesize that such high levels of atmospheric oxygen made possible the evolution of giant flying insects. 

Because O₂ is a highly reactive compound, its appearance posed challenges and provided opportunities to many organisms.  O₂ can be toxic and can also be used to extract the maximum amount of energy from food.

Around 10⁹ years ago, trace fossil burrows appeared; these  were likely to have been produced by simple worm-like, macroscopic (meaning larger) metazoans 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 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.

• How do scientists estimate the age of the universe?
• Why are fossils only found in (originally) sedimentary rock?
• Why are only a few organisms fossilized on death?
• What factors would influence the probability that a particular organism, or type of organism, would be fossilized? 
• If life first arose ~3.5 to 3.0 x 10⁹ years ago, why did it take so long for complex multicellular organisms to form?
• What kinds of adaptations would reflect the appearance of predators?
• What would evidence for pathogens look like in the fossil record?  When do you think pathogens first evolved?

The scientific study of life's 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 tradition differ as to the details of this event(s), but since the process is supernatural it cannot, by definition, be studied scientifically.

"Intelligent design" creationists claim that we can identify those aspects of life that could not have been created 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 proven wrong (often 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 an alien vacation visit! 

Unfortunately, this does not really answer the question of how life began, since those aliens also had to come from somewhere. 

This view was overturned 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. 

A first approach to understanding what is involved in the origin of life is to attempt to create living systems or their precursors in the laboratory.

A 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 then passed electricity through their experimental atmosphere.  

After awhile (hours/days), they found that a complex mix of compounds had been synthesized. Included in this mix were many different types of amino acids, the building blocks of proteins.

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 these systems were originally associated with the surface of specific minerals, from which they extracted energy and which could serve as catalysts for important reactions. 

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

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

First, because of the boundary, changes that occur within one such structure are not "shared" with neighboring systems.  Such systems can also "divide" in a crude way by fragmentation.

If changes within one system improves its stability, ability to accumulate resources, or efficiency of growth, 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

• What are the properties of life? What would you consider alive?
• Does Wöhler's synthesis of urea mean that all organic molecules can be easily synthesized in vitro?
• Would it be a good way to forward a scientific career to attempt to prove that a particularly complex molecule cannot be synthesized in the lab?  
• How is the idea of  "vitalism" similar to "intelligent design creationism"?