CONCEPTS IN BIOLOGY

PART V. THE ORIGIN AND CLASSIFICATION OF LIFE

19. The Origin of Life and the Evolution of Cells

19.2. Current Thinking About the Origin of Life

Today, it is clear that current living things are the result of biogenesis. New living things come from other living things as a result of either sexual or asexual reproduction. But that does not answer the question of how the first living thing developed. Because the evidence is strong that the universe had a time of origin, life must have originated spontaneously from nonliving material at least once. It is clear that we will never really know how the first living thing developed, because we cannot repeat the “original” experiment that led to the first living thing. However, that does not prevent scientists from speculating on the origin of life and evaluating relevant scientific information. There are many different hypotheses that have some supporting evidence.

An Extraterrestrial Origin for Life on Earth

Early in the 1900s, Swedish scientist Svante Arrhenius popularized the idea of panspermia. Panspermia is the concept that life arose outside Earth and that living things were transported to Earth to seed the planet with life. However, this idea does not explain how life arose originally. It sees Earth as similar to one of Spallanzani’s or Pasteur’s open flasks. Although Arrhenius’s ideas had little scientific support at that time, his basic concept has since been revived and modified as a result of new evidence gained from examinations of meteorites and space explorations.

Evidence from Meteorites

Since all life forms that we know about are based on organic molecules, the presence of organic molecules in space and in extraterrestrial objects such as meteorites suggest that life or the conditions necessary for life may have existed in other worlds.

In 1996, a meteorite found in Antarctica generated excitement about the possibility of life on other planets. Its chemical makeup suggested it had been a portion of the planet Mars that had been ejected from that planet as a result of a collision with an asteroid. Analysis of the meteorite showed the presence of complex organic molecules and tiny, microscopic objects that were thought to be ancient microorganisms. While scientists no longer think these objects are microorganisms or were formed by living things, many still think conditions on Mars may have been able to support life in the past.

Evidence from Mars and Beyond

In June and July 2003, two spacecrafts were launched by the National Aeronautics and Space Administration (NASA) to explore the surface of Mars. One of the important goals of these missions is to search for signs of present or past life. The robotic rover vehicles from these two spacecrafts have gathered much information about the surface of Mars. One important piece of information is that it is highly likely that in the past, liquid water existed in large enough quantities to form rivers, lakes, and perhaps salty oceans. In 2009 it was discovered that methane (a simple organic molecule) exists on Mars. However, methane can be produced by either geochemical or biological means and, at this point, it is not clear which processes are responsible.

In recent years, astronomers have been able to detect the presence of planets in other solar systems. By 2010 over 400 such planets have been identified orbiting stars in other solar systems. Although many are large gas planets like Jupiter, a few appear to be smaller rocky planets like Earth or Mars.

Although none of these discoveries proves that life exists or existed elsewhere in the universe, they keep open the possibility that life may have originated elsewhere and arrived on Earth.

An Earth Origin for Life on Earth

A popular alternative explanation for the origin of life on Earth focuses on chemical evolution. This hypothesis proposes that chemical reactions between simple, inorganic chemicals produced complex, carbon-containing organic molecules. These organic molecules, in turn, combined to form the first living cell. Furthermore, it is possible to perform experiments that test scientists’ hypotheses about these processes. Several pieces of evidence support the idea that life could have arisen on Earth.

1. The temperature range on Earth allows for water to exist as a liquid on its surface. This is important, because water is the most common compound of living things.

2. Analysis of the atmospheres of the other planets in our solar system shows that they all lack oxygen. The oxygen in Earth’s current atmosphere is the result of photosynthesis by living organisms. Therefore, before there was life on Earth, the atmosphere probably lacked oxygen.

3. Experiments demonstrate that organic molecules can be generated in an atmosphere that lacks oxygen.

4. Because it is assumed that all of the planets have been cooling off as they age, it is very likely that Earth was much hotter in the past. The large portions of Earth’s surface that are of volcanic origin strongly suggest a hotter past.

5. The discovery of specialized Bacteria and Archaea that live in extreme environments of high temperature, high salinity, low pH, or the absence of oxygen suggests that they may have been adapted to life in a world that is very different from today’s Earth. These kinds of organisms are found today in unusual locations, such as hot springs and around thermal vents in the ocean floor, and may retain characteristics that were essential to the first organisms formed on the primitive Earth.

19.2. CONCEPT REVIEW

4. Provide two kinds of evidence that support the idea that life could have originated on Earth.

5. Provide two kinds of evidence that support the idea that life could have arrived on Earth from space.