Requirements for Life One of the biggest questions in astronomy is whether life exists elsewhere in the universe Before we discuss the possibility of life elsewhere, we must have a better understanding of life itself Today will we discuss the 'definition of life' and the requirements needed for life to survive What is Life? Definition of Life 1
Definition of Life Life Example Non-Life Example The Gray Area There are some instances where the question of living or non-living is hard to answer Viruses are complex organic entities which sometimes exhibit the characteristics of life In non-living surroundings, like a pool of water, a virus is just lifeless matter If the virus finds a host living organism, it goes into action interacting and reproducing Polio virus Life As We Know It Most of the life on Earth follows the guidelines below: Primarily carbon-based lifeforms Carbon, hydrogen, oxygen, and nitrogen (CHON) Requires liquid water for internal transport Transports fuel, food, waste etc... Requires some sort of energy In most cases, the original source of energy is the Sun Has evolved from simple to complex 2
There Are Exceptions In the past few decades, numerous organisms called extremophiles have been found These organism live and thrive under conditions where most other organisms would die These creatures have been found in deep ocean volcanic vents, sulfur hot springs, Antarctic ice, and deep within the crust Many of these places resemble other bodies in the Solar System Timeline of Life on Earth Years Ago 4.60 Gyr solar system formation, giant impact creates Moon 4.41 Gyr oldest rocks on Earth (< 5% of Earth crust more than 3 Gyr old) 4.00 Gyr Sun shines at 0.7 L o ; day 18 hours; RNA invented? 3.52 Gyr oldest reliable chemical evidence of life (kerogen deposits) 3.46 Gyr oldest claims of fossil life (cyanobacteria in Australia) 3.5-2.0 Gyr O 2 builds up in Earth atmosphere Years Ago Timeline of Life on Earth 2.70 Gyr good evidence of all three branches of tree of life 2.60 Gyr oldest reliable fossil evidence of life (dividing cells in stromatolites) 2.00 Gyr eukaryotes (which have a cell nucleus) appear 1.20 Gyr sex invented, variations become rampant; multicellular life appears 0.60 Gyr explosion of life forms (beginning of Cambrian Period) 0.25 Gyr Permian-Triassic extinction wipes out 90% of animal species 0.065 Gyr KT event(s) wipes out 50% of animal species 3
Five Steps to Life on Earth I. COLLECT MATERIALS II. MAKE/GET AMINO ACIDS III. MAKE RNA and DNA IV. MAKE REPRODUCTIVE FORMS V. EVOLVE LIFE FORMS Step One: Collect Materials Remember, Earth's first atmosphere was mainly hydrogen and helium, which was easily lost Volcanoes, comets, and meteorites provided lost of water, methane, ammonia, carbon dioxide, sulfur dioxide, etc. These molecules were broken down by UV sunlight, leaving many nitrogen, oxygen, carbon behind Step Two: Make Amino Acids Amino acids are basic organic molecules which combine to form proteins Proteins are the most abundant and important molecular structures in all lifeforms - the building blocks of life The question is, how did these complex organic molecules ever form in the first place? 4
The Miller-Urey experiment in 1953 sought to reproduce conditions of a young Earth Life from Non-life An 'atmosphere' of water, ammonia and methane is created in a test tube An electrical charge is passed through the gas to simulate lightning This and other similar experiments confirmed that amino acids can easily be formed under these conditions Life from Space - Panspermia Another interesting but controversial theory is that life may have originated from space Some scientists believe conditions on Earth may not have been well suited for life Instead, they believe that compounds in interstellar dust can form amino acids when exposed to UV sunlight There is evidence of organic compounds in some interstellar dust clouds, in some meteorites, and in comets Step Three - Make RNA and DNA Both RNA and DNA are extremely complex biological molecules It is hard to imagine how such complex arrangements of molecules were ever created Many believe that DNA was first formed in some sort of crystal Cold?: Ice crystal lattice Medium?: Clay crystal lattice Hot?: Volcanic crystal lattice 5
Step Four: Reproduce While it may sound obvious, life HAD to find a way to reproduce itself, otherwise there would have been just many one-time, short-lived organisms The first type of reproduction was asexual, where one individual can form an offspring, like a bacterial cell splitting The creation of sexual reproduction led to a huge diversification of lifeforms, due to the increased mixing of genetic material The final step for life is to diversify and spread through evolution Step Five: Evolve A single species of life is too vulnerable to changes in environment Through competition and interaction, life became an unstoppable force on Earth Without evolution, life would have been eliminated long ago The Tree of Life 6
Biodiversity Over Time We have only catalogued about 1,500,000 species Insects rule! The estimated rate of extinction is much higher than the rate of discovery Find them before they are gone! Diversity of Life 7