1.1. First theories

Unit 3: The origin and evolution of life 1 2 3 4 5 6

1. The origin of Life

1.1. First theories about the origin of life

a) Creationism

In this theory the origin of life is attributed to one or more Superior Beings (Gods). All religions have stories about the origin of living beings. In fact, creationism is not a scientific theory because neither the existence of this supernatural being nor its implication in the origin of life could be demonstrated.

b) Spontaneous generation

Since ancient times the idea of spontaneous generation of living beings was considered acceptable. According to this theory, life could be appeared from water, mud or by combination of the four fundamental elements: air, water, fire and earth.

Aristotle proposed the spontaneous origin for worms, insects and fishes from substances like sweat, dewdrops, and moisture.

According to him, the transformation in living beings was the result of the interaction of this inert matter with a force able to give life. He called to this force pneuma (soul).

This theory was in force for a long time. In 1667, Johann B. van Helmont, Dutch medicine doctor, proposed a recipe that allowed the spontaneous generation of mice:

"Creatures such as lice, ticks, fleas and worms are our hosts and neighbours, but born from our entrails and excrements.

If we put sweated underwear and wheat into a container of wide open, 21 days later the smell changes and goes through the wheat shells changing the grains into mice.

However the most important it is that these mice are of two sexes and can cross with other mice appeared in normal way…”

Some scientists, however, were not agree with these explanations and began to subject to experiments all these ideas and theories.

Franchesco Redi, Italian medicine doctor, realized the first experiments to demonstrate the falseness of spontaneous generation.

He got prove that the worms that infected meat was larvae that came from eggs lay by flies on it.

He designed this simple experiment. He put three pieces of meal in three different pots. The first one was closed hermetically, the second pot was covered with gauze and the third pot kept open. Several days later, Redi discovered that into the closed pot do not be worms although the meal was decayed. On the gauze that covered the second pot were eggs (flies cannot go through the tissue to lay the eggs on the meal). Finally the third pot contained a large amount of larvae and flies. In this way, it kept demonstrated that flies do not come from the meal, but from the other flies.

At the end of 17^th century, Anton van Leeuwenhoek, thanks to the improvement of optical microscope, discovered an unknown world. Looking at pond water through microscope, he found large amounts of microorganisms that seemed appear suddenly. This discovery reinforced the spontaneous generation hypothesis again. In spite of Redi’s experiments, the theory had not been entirely refuted, because his investigations demonstrated the origin of flies but not the origin of other organisms.

In the same time, other scientist, John Needhad, maintained the existence of a vital force that was the origin of life. His suppositions were based in his experiments. He boiled ox soup in a bottle and then closed it with a cork. Several days later by analysing the content of the bottle he found microorganisms. If the heat transmitted to the soup was enough to kill any organism, the presence of living beings had to be produced by the vital force.

Nevertheless, Lazaro Spallanzani was not agreed at all. He repeated the same experiment but he closed hermetically the bottles and then put them to boil. In no case he found microorganisms in the soup. He deduced that the microbes found by Needhad came from the air that penetrated through the cork.

c) Biogenesis

In 1862, Louis Pauster, French medicine doctor, carried out a serial of experiments in order to solve the problem of spontaneous generation. He though that the microorganisms in the air were responsible for the putrefaction of organic matter.

To demonstrate his hypothesis, he designed a special flask with s-shape neck, that he called swan neck flask. He put nutritive dissolutions into the flasks and then he boiled them until sterilised them. After several weeks the soup had not decayed. The microorganisms kept in the curve of the flask neck and did not contaminate the liquid. To verify his observations, he cut the neck of the same flasks. In all of them the liquid was polluted.

In this way, the falseness of spontaneous generation was absolutely demonstrated and the biogenesis hypothesis was accepted: “Every living being comes from other living being”

Animation: Pasteur's experiment (W. H. Freeman)

Interactive: Redi and Pasteur experiments (Pearson)