The Development of Multicellular Organisms

After a long while, individual eukaryotic cells probably made the same "discovery" made much earlier by the individual replicators. They would come to work in teams, each cell contributing to the good of the team just as the replicators had. In time, they might combine genetic material to yield multicellular organisms. In these organisms, all the genetic material of the entire organism is present in every cell, but only certain genes are turned on in each cell, giving rise to cell specialization.

Further Specialization and Development

From this point, it is fairly simple to imagine the general idea of the rest of the story of evolution: multicellular organisms branched and diversified to occupy new ecological niches, adapting as they went to their new environments. As these diversifications took place, the multicellular organisms increased in complexity in proportion to the demands of the environment. Eyes, skeletons, nervous systems, and backbones, as well as numerous other adaptations, all arose through gradual selection of mutations. As competition increased, species continued to diversify, especially in areas where population density was high. Life eventually moved out of the ocean and ventured onto land, where more and more species were formed. Eventually, extremely large and complex organisms evolved. Finally, one of the more advanced groups - the primates - emerged, and with it eventually came the human species.

Alternative Theories

Much of this timeline of life is hypothetical; it relies on scientists' best estimates of the conditions on the early Earth and of the likelihood of the formation of complex organic molecules. There are some scientists who are dissatisfied with the above view; they have proposed numerous alternative theories regarding the origin of life.

One such theory is a relatively minor modification of the above scenario. Taking into account calculations demonstrating the enormous odds against the spontaneous formation of complex organic molecules, some scientists propose that the first such molecules to exist on Earth did not form here, but rather were introduced by way of meteorites colliding with Earth. These scientists postulate that organic molecules are rarer than normally thought, and have only arisen a relatively few times in the course of the universe's long history. (Other arguments state that more time was available elsewhere in which the molecules formed, or that conditions elsewhere were more conducive to their formation.) Somehow, the organic molecules formed in some distant location found their way onto rocks that happened to be on a collision course with Earth. This theory has been called panspermia.

Another theory often called panspemia states that life did not originate on Earth, but rather was "seeded" by life forms from space, riding on comets or other celestial objects that collided with Earth during its early history. This theory suggests that life is much more difficult to get started than was previously thought, but once it has been started it will spread much further. Again, the theory relies on spontaneous generation of organic molecules at some point in the universe's history, but eliminates the need for this unlikely event to have occurred during the history of our planet. Rather, the entire time in the universe that planets have existed is available for the generation of such molecules; however, there remains the problem of how the molecules formed life, got onto a celestial object, and managed to survive the impact of its collision. Francis Crick, co-discoverer of DNA, has discussed both ideas, and Fred Hoyle has promoted panspermia in the first sense.

Cautions and Remarks

As stated above, many aspects of the timeline presented are purely theoretical. The early atmosphere is now thought to be mush less conducive to the formation of complex molecules than was originally supposed, and "origin-of-life" chemistry has so far yielded few concrete results. Even a molecule such as protein or RNA would be unlikely to arise spontaneously from the "primordial soup", and the full-fledged complexity of the modern DNA-RNA-protein system would be incalculably unlikely to arise. However, research into the chemical basis and beginning of life continues with vigor, and new theories on the origin of life on Earth may yet revolutionize our understanding of this momentous event.

Express Yourself!

Do you have a dissenting view or an alternative theory about the origin of life on earth? Please feel free to share your opinions with other users - just visit the Replicators Forum!

Looking Further: Links and References

These links and references will be of assistance in the study of the origin and history of life on Earth.

• Books
  • The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design by Richard Dawkins
  • The Selfish Gene by Richard Dawkins

• Websites
  • The Tree of Life
  • Probability of Abiogenesis Calculations
  • The Probability of Abiogenesis

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