Lesson Objectives:- Astrobiology
- The earliest life on Earth
- Theory of evolution
- Requirements for life
Astrobiology is the scientific search for life in the universe.
Astrobiology aims to understand the origin and evolution of life on Earth so that we can learn the conditions under which life could arise and evolve elsewhere. That will then allow us to search for new worlds, both in our solar system and beyond, that might have conditions for life. Finally, astrobiology deals with looking for evidence of actual life on other worlds.
When did life arise on Earth? Life arose quickly and easily on Earth. This indicates that life could arise easily on Earth-like worlds.
We have learned a lot about Earth's origins from fossils. Fossils are relics of organisms that lived and died a long time ago. They often formed when dead organisms fell to the bottom of the sea and got buried by sediment layers. Sediments formed from erosion from land that was carried to the sea by rivers. Over millions of years, sediments piled up on the seafloor and the weight of the upper layers compressed underlying layers into rock.
Erosion can often expose fossils. Some places like the Grand Canyon have sedimentary layers that have recorded hundreds of millions of years of Earth's history.
Geologists use the layers of rocks and fossils to determine Earth's history across a 4.5 billion year time scale that we call the geological time scale. We have more information on more recent periods on this scale due to the accessibility of fossils. It is difficult to see evidence of life prior to 3.5 billion years ago because the farther back in time we go, the more likely rocks are to have undergone significant transformation, meaning fossils are no longer intact. Carbon isotopes, however, used to test fossils for isotopes associated with the presence of life, indicate that life on Earth may have existed as much as 3.85 billion years ago.
The word evolution just means to change with time. The biologist Charles Darwin came up with the theory of evolution in 1859 to understand why species undergo change.
Fossil records provide evidence that evolution has occurred. Darwin's theory explains why. He studied the relationships between living species and fossils of extinct species, specifically in the Galapagos Islands. He put forth a model to explain behavior. First, populations can produce more offspring in a local area as long as the environment supports it with enough food and shelter. This leads to overpopulation where there is competition for survival among individuals. Secondly, individuals can vary in heritable traits and no two individuals are exactly alike. Third, in the struggle for survival, individuals with advantageous traits that help them survive and reproduce leave more offspring.
Darwin called this last step natural selection. Natural selection is the process by which organisms retain genetic traits that provide reproductive advantages.
Darwin's theory of evolution explained why certain traits are passed on and helps us understand the various stages in the evolution of any species. However, it does not explain how. A scientific look at the DNA of an organism does. DNA, or deoxyribonucleic acid, is the molecule in an organism that passes on genetic traits.
Living organisms reproduce by copying DNA and passing them on to their descendants. Evolution happens because the transfer of genetic information is not perfect.
Mutation is a change in the organism's DNA. All living organisms evolved from a common ancestor many years ago. Scientists theorize that early organisms arose near a source of chemical energy such as deep sea volcanic vents. Lab experiments show that molecules of life form easily under conditions that match early Earth conditions.
The earliest life on Earth detected by scientists is cyanobacteria, single-celled organisms that release oxygen through photosynthesis that date as far as back as 2.7 billion years ago. After hundreds of millions of years, they produced enough oxygen to persist and accumulate in the air. For a long time, living organisms on Earth were single-celled with larger plants and animals only arising in the last few hundred million years.
What are the necessities of life? There are 3 basic requirements:
- a source of nutrients (this includes atoms and molecules from which an organism can build living cells)
- energy (to fuel life activity)
- and liquid water
Organic molecules are present almost everywhere, even on comets. But liquid water is not and that seems to pose the biggest restraint to life on other planets.