Category Archives: evolution

Life’s a Tough Cookie

It Survives Bombardments

The early Earth, moon, and bombardment. Artwork by D. Aguilar.
The early Earth, moon, and bombardment. Artwork by D. Aguilar.

Our planet is young, in cosmic time. And life on our planet is just about as young.

The universe is some 13.7 billion years old. A lot of time had to pass — say, nine of so billion years — before the Earth began forming, some 4.5 billion years ago.  The place was nothing like the planet we know today. It had just accreted from “stuff” in the proto-solar nebula. The baby Earth was hot, but cooling down. It had some kind of atmosphere, although nothing we could breathe.  And, it was being hammered by leftover debris from the rest of the solar nebula. The period it was experiencing is called the Late Heavy Bombardment, and it was long thought that the bombardment would have sterilized the surface of the planet (if any life had managed to arise there).

It turns out that the picture of a spanking clean new planet with NO life on the surface during and after the bombardment may need to be rejiggered a bit. A study funded by NASA indicates that the Late Heavy Bombardment may not have sterilized the early Earth as completely as scientists thought, and that some of the incoming asteroids (some the size of Kansas) might have actually boosted the chances for life.

The study focused the Late Heavy Bombardment, which occurred approximately 3.9 billion years ago. It pummeled the planet anywhere from 20 to 200 million years. In a letter published in the May 21 issue of Nature magazine titled “Microbial Habitability of the Hadean Earth during the Late Heavy Bombardment,” Oleg Abramov and Stephen J. Mojzsis, astrobiologists at the University of Colorado’s Department of Geological Sciences, described a computer modeling project they designed to study how the bombardment heated Earth.  They ran simulations of the bombardment and the results show that while the Late Heavy Bombardment might have generated enough heat to sterilize Earth’s surface, it probably didn’t do much damage to microbial life in subsurface and underwater environments. In fact, those little critters almost certainly would have survived the bombardment without much trouble.

The story of life on this planet is a tough one to tell. For one thing, it’s not easy to say exactly when life arose.  Scientists are getting closer to pinpointing its time, but we may never know exactly where it got the first “oomph” that transformed some randomly mixing chemicals into a living thing.

The sort of “canonical” start date that we toss around is usually 3.8 or 3.9 billion years ago, but it could well have been earlier. These findings are significant because they indicate that if life had begun before the LHB or even earlier than 4 billion years ago, it could have survived in those hidden places protected by the surface from the bombardment.  Certainly all the elements for life were in place by the time the planet finished forming — carried in by asteroids and comets, and in place from the elements from which the planet formed.

Astrobiologists are examining each step in the ladder of life minutely — from the elements that formed this planet to the processes taking place on and near Earth during the crucial time when life arose. What they learn may well translate to the stars, especially when we start looking at other planets where life may have arisen.

American Astronomical Society Statement on the Teaching of Evolution

The American Astronomical Society supports teaching evolution in our nation’s K-12 science classes. Evolution is a valid scientific theory for the origin of species that has been repeatedly tested and verified through observation, formulation of testable statements to explain those observations, and controlled experiments or additional observations to find out whether these ideas are right or wrong. A scientific theory is not speculation or a guess — scientific theories are unifying concepts that explain the physical universe.

Astronomical observations show that the Universe is many billions of years old (see the AAS publication, “An Ancient Universe,” cited below), that nuclear reactions in stars have produced the chemical elements over time, and recent observations show that gravity has led to the formation of many planets in our Galaxy. The early history of the solar system is being explored by astronomical observation and by direct visits to solar system objects. Fossils, radiological measurements, and changes in DNA trace the growth of the tree of life on Earth. The theory of evolution, like the theories of gravity, plate tectonics, and Big Bang cosmology, explains, unifies, and predicts natural phenomena. Scientific theories provide a proven framework for improving our understanding of the world.

In recent years, advocates of “Intelligent Design,” have proposed teaching “Intelligent Design” as a valid alternative theory for the history of life. Although scientists have vigorous discussions on interpretations for some aspects of evolution, there is widespread agreement on the power of natural selection to shape the emergence of new species. Even if there were no such agreement, “Intelligent Design” fails to meet the basic definition of a scientific idea: its proponents do not present testable hypotheses and do not provide evidence for their views that can be verified or duplicated by subsequent researchers.

Since “Intelligent Design” is not science, it does not belong in the science curriculum of the nation’s primary and secondary schools.

The AAS supports the positions taken by the National Academy of Sciences, the American Association for the Advancement of Science, the National Science Teachers’ Association, the American Geophysical Union, the American
Chemical Society, and the American Association of Physics Teachers on the teaching of evolution. The AAS also supports the National Science Education Standards: they emphasize the importance of scientific methods as well as
articulating well-established scientific theories.

Suggested reading: The Ancient Universe: How Astronomers Know the Vast Scale
of Cosmic Time.

Published by the American Astronomical
Society.