Life in the Universe

Life on Earth

Scientists learn about past life on Earth by studying fossils buried deep in the Earth’s crust. Fossils are created when dead organisms fall to the bottom of a body of water, and over the course of many millions of years, sediment piles up on top of that dead organism, creating fossils buried deep in rocks. The rocks or sediments create fossils that are later revealed by tectonic plate activity or erosion (Voit, 2015).

Determining the age of these fossils is how scientist determine how long life has existed on Earth. The relative age of fossils is easy to determine, the deeper a fossil is buried, and the older it is. However, radiometric dating confirms the relative age of fossils, giving a precise age for fossils found buried deep under water (Voit, 2015). The geological time scale is a measurement of intervals here on Earth, and its helps divide the four and a half billion years into more manageable eras for scientists.

The further back on the geological time scale scientist look the harder it is to identify fossils. The reason for that is because older rocks are harder to find than younger rocks, when older rocks are found, they have often gone through transformation caused by heat and pressure, that would have destroyed any fossil evidence, and third because nearly all life prior to a few hundred million years ago all life on Earth was microscopic and microscopic fossils are harder to identify (Voit, 2015). However, despite the difficulties in finding old rocks, geologists have found evidence in rocks called stromatolites, that suggests life started around three and a half billion years ago, and that older, more microscopic, simpler, single cell life evolved even earlier than that (Voit, 2015).

Fossil evidence of life before three and a half billion years ago is not likely to be found. However, the oldest sedimentary rock found on Earth, on the island of Akilia, near Greenland, are cut through with volcanic rock that radiometric dating dates back to 3.85 billion years ago, suggesting that the sediment is even older (Voit, 2015). The carbon isotopes in these rocks suggest that they may once have held living organisms. Carbon has two stable isotopes, carbon-12 and carbon-13. Living organisms incorporate carbon-12 more easily than carbon-13. All fossils have a lower fraction of carbon-13 than rocks that contain no fossils. Therefore, scientists conclude that these rocks, found on Akilia, must have, 3.85 billion years ago, contained life, and not only did it contain life, but that life must have been pretty widespread because the chances of Scientist’s finding the evidence otherwise, is highly unlikely (Voit, 2015). Due to this evidence of early life, scientists believe that life probably arose pretty easily here on Earth.

The Genesis of Life

Fossil records prove that life has gone through many changes in the last 3.85 billion years. These changes can be described as the theory of evolution, first put on paper by Charles Darwin in 1859 (Voit, 2015). Although the theory of evolution has gone through some major religious battles since it was first published, it is a fact that evolution occurred on Earth. Evolution simply means to change with time, and fossil records indicate that life has changed many times over the years.

Darwin built his proof for the theory of evolution around two undeniable facts and one inescapable conclusion.  Fact 1: overproduction and competition for survival. Any localized population of a species has the potential to produce far more offspring than the local environment can support with resources such as food and shelter. This overproduction leads to a competition for survival among the individuals of the population (Voit, 2015). Fact 2: individual variation. Individuals in a population of any species vary in many heritable traits (traits passed from parents to offspring). No two individuals are exactly alike, and some individuals possess traits that make them better able to compete for food and other vital resources (Voit, 2015). The inescapable conclusion: unequal reproductive success. In the struggle for survival, those individuals whose traits best enable them to survive and reproduce will, on average, leave the largest number of offspring that in turn survive to reproduce. Therefore, in any local environment, heritable traits that enhance survival and successful reproduction will become progressively more common in succeeding generations (Voit, 2015).

Natural Selection

Darwin called the unequal reproductive success of some species over others natural selection. Natural selection is when traits that give an advantage to the species is selected over less advantageous traits. Over time, natural selection can help individuals of a species become better able to compete for scarce resources. If enough small individual variations accumulate, natural selection can even give rise to an entirely new species (Voit, 2015).

DNA provides scientists with the ability to discover how life has changed on a molecular level. Living organisms reproduce by copying DNA and passing this DNA onto its children. A molecule of DNA consists of two long strands wound together in the spiral shape known as a double helix. The instructions for assembling a living organism are written in the precise order of four chemical bases (abbreviated A, T, G, and C for the first letters of their chemical names) that make up the interlocking portions of the DNA “zipper.” These bases pair up in a way that ensures that both strands of a DNA molecule contain the same genetic information (Voit, 2015).

Evolution occurs because the transfer of genetic information from one generation to the next is not always perfect. An organism’s DNA may occasionally be altered by copying errors or by external influences, such as ultraviolet light from the Sun or exposure to toxic or radioactive chemicals. Any change in an organism’s DNA is called a mutation. Many mutations are lethal, killing the cell in which the mutation occurs. Some, however, may improve a cell’s ability to survive and reproduce. The cell then passes on this improvement to its offspring (Voit, 2015).

Three and a Half Billion Years of Evolution

Earth began about four and a half billion years, the moon formed soon after that, and the Earth developed its first ocean about 4.3 billion years ago. Scientists believe that oceans are where life first developed. Once life took hold, about 3.85 billion years ago, evolution quickly changed it so that the hardiest of the species stayed and continued to evolve. However, despite the quick hold of evolution, living organisms remained single-celled for about a billion years after life took hold (Voit, 2015).  Once oxygen took hold on the surface, and an atmosphere developed, evolution brought life out of the ocean.

Nearly all the oxygen in our atmosphere was originally released through photosynthesis by single-celled organisms known as cyanobacteria. Fossil evidence indicates that cyanobacteria were producing oxygen through photosynthesis by at least 2.7 billion years ago, and possibly for hundreds of millions of years before that (Voit, 2015). Oxygen took hundreds of millions of years to accumulate in the Earth’s atmosphere, reaching levels humans could breathe only a few hundred million years ago (Voit, 2015).

While today we think of oxygen as a necessity of life, in truth, oxygen was probably poisonous to most life on Earth before about two billion years, and remains poisonous to some micro-organism today. Oxygen probably provided tremendous pressure for the evolutionary process to excel, and was probably a major contributor to complex life of plants and animals (Voit, 2015). (Life on Earth)

About 542 million years ago, or an age known as the Cambrian period, evolution changed animal life from primitive microbes into the basic body type we still see on Earth today. This evolutionary process occurred in a relatively short time frame, and is often referred to as the Cambrian explosion (Voit, 2015).

Early dinosaurs and mammals evolved about 225 to 250 million years ago, but dinosaurs proved more evolutionarily successful and ruled the Earth for over a 100 million years. The catastrophe that killed the dinosaurs 65 million years ago, paved the way for larger mammals and ultimately for us humans (Voit, 2015). Humans only arose on the scene about a few million years ago, or after 99.9 percent of Earth’s history had already occurred. The history humans have of technology has only been about 99.99999 percent of Earth’s history (Voit, 2015).

Possibility of Life in the Universe

Scientists have made some predictions about the requirements for life in the Universe. The first necessity of life is that a planet reside in the habitable zone of its star. Once the requirement is met, scientists have determined that life has only three basic requirements: A source of nutrients (atoms and molecules) from which to build living cells, energy to fuel the activities of life, whether from sunlight, from chemical reactions, or from the heat of Earth itself, and liquid water (Voit, 2015). Liquid water is the only requirement that is not common in the universe. (Potential Habitable Planets)

Habitability also requires an atmosphere and a climate that remains stable over long periods of time. Other planets must be at least as large as Earth and have ongoing volcanism and plate tectonics (Voit, 2015). A global magnetic field is also necessary for a planet to maintain a habitable surface over long periods of time (Voit, 2015). Based on everything we have discussed so far, life should be abundant in the universe.

The Search for Extraterrestrial Life

The search for intelligent life in the universe is known as SETI or the search for extraterrestrial intelligence. The Drake Equation summarizes the factors that determine the number of civilizations in our galaxy that we could potentially communicate with (Voit, 2015). The number of habitable planets in the galaxy by the fraction of habitable planets that actually have life, by the fraction of planets where civilization has had time to evolve, by the number of planets that actually have a civilization now, and not millions or billions of years in the past (Voit, 2015). Scientists do not actually know any of the values for the Drake equation, but it is a way to organize their thoughts.

Most SETI research facilities use large radio telescopes to search for radio signals in space being transmitted from alien worlds (Voit, 2015). Humanity has been broadcasting strong radio signals into space since about the 1950’s, and scientist reason that if any intelligent life has developed out there, that they would communicate much the same way that we do. Essentially, any civilization out there that could pick up our signals could watch television ads from the 1950’s, that civilization would have to be no more than sixty light years away however, and have much stronger telescopes than humans currently have at their disposal (Voit, 2015).

Humans have attempted to send signals out into space for any intelligent life that could pick it up, but so far we have not made too many attempts, and nothing we have sent out would have traveled far enough to reach another civilization. The signal sent out in 1974 will not reach M13, the star it was sent out to, for another 25,000 years, and then another 25,000 years for an answer to reach us, if there is intelligent life on M13 (Voit, 2015).

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