SC160 Basic Biology
Directions: Be sure to make an electronic copy of your answer before submitting it to
Ashworth University for grading. Unless otherwise stated, answer in complete sentences,
and be sure to use correct English spelling and grammar. Sources must be cited in APA
format. Your response should be one (1) single-spaced page in length; refer to the
"Assignment Format" page for specific format requirements.
On page 410 of your textbook, there is a summary statement concerning what scientists
believe about the origin of life on earth.
Summarize the three (3) current competing theories of the origin of life on Earth: it
arrived from an extraterrestrial source, it originated as a heterotroph, it originated as an
autotroph.
On the Origin of Terrestrial Life
The question of how life originated on our planet is not yet completely answered.
While there is clear evidence that life on Earth is at least 3.5 billion years old, the exact
genesis and pathway of life on Earth is still not fully understood. As described by Enger,
et al. (2007), there are three leading hypotheses about the origin of life, all of which
generally support the observational evidence, but none of which has as of yet been
determined to be the unequivocal answer. The first hypothesis is called the
extraterrestrial hypothesis. In this scenario, life originated from an extraterrestrial source
and was deposited on Earth, shortly after the formation of the planet, by a comet or
asteroid impact (or perhaps multiple such objects). The other two hypotheses posit
terrestrial or “home grown” origins—that life arose in-place, from specific precursors
fostered by the environmental conditions on primordial Earth. These terrestrial
hypotheses are divided into the heterotroph and autotroph hypotheses, named for the
type of organisms thought to be the earliest life forms to arise. Briefly, heterotrophs
consume other organisms or organic compounds for energy, while autotrophs get energy
from sunlight, inorganic molecules and/or minerals, and generally synthesize organic
molecules.
First we consider the possibility of an extraterrestrial source. If the
extraterrestrial hypothesis is correct, early life forms evolved elsewhere in the solar
system and were transported here by comets. In some sense, this is begging the question,
as it does not directly address how life may have formed on these distant bodies.
However, we do know that most comets contain abundant water, perhaps the single most
important ingredient for life. During periodic close passes to the Sun, the temperatures
can rise to levels conducive to biological processes. We also know that during the early
years of the Earth, there were frequent impacts from comets. In fact, it has been
suggested that the majority of the water on Earth today came from comet impacts in the
early history of the planet. Still, there is no incontrovertible evidence that this is the
initial mechanism for life arising on Earth.
Next we examine the two terrestrial hypotheses. In the heterotroph hypothesis,
the first organisms would have fed on the organic substances thought to be abundant on
primordial Earth. Since at this point the atmosphere was without oxygen, such
heterotrophs would have utilized completely anaerobic metabolisms, producing CO2 as a
byproduct. Eventually, they would eat their way through their food supply, but during
this phase the increase in CO2 and decrease water vapor in the atmosphere would have
paved the way for autotrophs who would use sunlight as fuel, and themselves become a
food source for future heterotrophic organisms. The original heterotrophs would have
found the sunlight and increasing O2 concentrations poisonous, and remain only in
secluded niches.
While the heterotrophic hypothesis is arguably the predominant viewpoint today,
it is plausible that the first organisms were in fact autotrophs. In the autotroph hypothesis,
the earliest living organisms were autotrophs. Specifically, these would have had to be a
type of autotrophs called chemotrophs, meaning they derived their energy from inorganic
chemical compounds, as opposed to sunlight, which was absent in this early period.
Eventually, a combination of heterotrophs and autotrophs emerged as the atmosphere
cleared, sunlight became abundant, and the concentration of oxygen increased.
In summary, there are two distinct possible pathways for the origin of life on
Earth: terrestrial and extraterrestrial. If Earth-life has a terrestrial origin, it is likely to
have been begun with the emergence of heterotrophic organisms, followed by autotrophs,
but the reverse may also be true. As of today, there is no irrefutable evidence sustaining
a particular pathway. Studies of biology, geology, physics, chemistry and even planetary
science all continue to add insights into the history of primordial Earth, helping unravel
this important question.
References
Enger, E. D., Ross, F. C., & Bailey, D. B. (2007). Concepts in biology (12th ed.). Boston:
McGraw-Hill.
Cowen, R., (2013). History of Life (5th ed.). West Sussex, UK: Wiley-Blackwell.