The History of Life on Earth

The History of Life on Earth An overview of current Scientific Theories

The Big Bang Before our solar system existed, there was an enormous area of gas and dust. Scientists now tend to believe that our solar system was created by an explosion within that cloud of gas and dust • After the explosion, the pure weight of cycling matter forced it to condense • The center came under so much pressure that a nuclear reaction started to take place. The hydrogen began to fuse to produce helium – the sun was formed • Other areas of dense cloud condensed to form the 9 planets that orbit the sun

The Big Bang The theory of the big bang can still be considered controversial and hence become a focus for scientific humor

The Early Atmosphere Artists impressions • Scientists don’t really know what the early conditions were like on the planet earth before life begin. • They believed that the gases in the early atmosphere consisted of CO2, CH4 (methane), H2 & NH3 (ammonia) • Here are some artists impressions of the earth before life began • What are the common features between the photos?

First Organic Molecules - 4 bya One theory 1: they came from Earth itself • One of the theories of the origin of life on earth was proposed by Haldane and Oparin. They suggested that organic molecules were built by the inorganic molecules that existed in the early oceans at the time • Electrical discharges were thought to be a catalyst for these reactions • The oceans became a kind of ‘organic soup’ Organic molecules produced from chemical reactions between inorganic molecules

This chamber represented the early atmosphere with both inorganic gases and electrostatic charges Condenser cooled the vapour into a liquid Sample collected contained organic chemicals; from this living things could be made The water here represented the early seas First Organic Molecules – 4 bya In the 1950’s two scientists, Urey and Miller, attempted to set up a laboratory experiment that simulated the conditions of the early atmosphere. Their set up looked like this…

First Organic Molecules – 4 bya One theory 1: they came from Earth itself • Recently scientists have conducted similar experiments to Urey and Miller • These scientists have used various combinations of all the inorganic molecules thought to be present in the early atmosphere • They have also used ultra violet radiation instead of electrical charges • With these experiments they have been able to produce a variety of amino acids, each different combination of inorganic molecules producing a different variety of amino acids

First Organic Molecules – 4 bya One theory 2: they came from outer space • Scientists believe that the early earth was heavily bombarded with meteorites • Certain type of meteorites that were first analysed in the 1970’s have been found to contain various organic molecules including amino acids • The Murchison meteorite (found near Murchison in Victoria) was shown to contain 92 amino acids – 19 of which were similar to those found on Earth • If organic molecules have come from outer space, what does this suggest about the possibilities of life on other planets within our, or other, solar systems?

The oceans were covered with molecules that had hydrophobic and hydrophilic ends During turbulance, a seaspray droplet becomes isolated from the main body of water. Various molecules are trapped inside the droplets The outer molecules of the droplets form a membrane around their molecular ‘inclusions’ and re-enter the ocean The molecules inside the membranes are protected from the external environment. They continue to produce new molecules inside the protective membranes Now known as a coacervate The First Membranes – 3.5/4 bya If molecules can protect themselves from the environment within a membrane, they are more likely to build other molecules or perhaps even replicate.

The First Prokaryote – 2/3 bya • For about 550 million years prokaryotes – bacteria - were the only living organisms. Even today they are the most prolific • The first cells were thought to be heterotrophs – feeding on other organisms • Prokaryotes then developed pigments that could trap the light from the sun and undergo photosynthesis – the autotrophs, or cyanobacteria, allowed the atmosphere to become oxic (containing oxygen) instead of anoxic (containing no oxygen) which it had been • An oxic environment meant that organisms could use the oxygen to produce energy in the process of cellular respiration. This way they could be more active • Heterotrophs continued to exist and fed off the autotrophs as well as other heterotrophs

The First Eukaryote – 1/2 bya • Eukaryotes were thought to have formed from two prokaryotes living symbiotically – the smaller one inside the larger one • Photosynthetic prokaryotes appear to have evolved into chloroplasts and then started to live inside larger cells to help them photosynthesize. • Energy producing prokaryotes appear to have evolved into mitochondria and then started living inside other cells to help give them energy to grow and carry out cellular functions • Mitochondria and chloroplasts both contain their own DNA, which supports the theory that they were once independent living organisms • The ‘new’ cells were also able to replicate their DNA and divide using mitosis and cytokinesis • These cells then went on to develop many kinds of organelles, such as golgi bodies & endoplasmic reticulum, not found in prokaryotic cells • Finally, eukaryotic cells started to reproduce sexually, mixing their genes and forming new cells genetically different to the parent cells

The First Eukaryote – 1/2 bya • This is another flow chart showing how eukaryotic cells developed from organic molecules that came together inside a membrane

First Colonial Organisms - 0.5/1.5bya • These photosynthetic singled celled prokaryotes are called Stromatolites • The stromatolites are a form of Cyanobacteria • They are found in the fossil record up to 3000 million years ago, and until recently were thought to be extinct • These stromatolites are currently growing in Shark Bay in Western Australia • The stromatolites trap sediment which they use to build the column, then they grow through the sediment to form a new layer on top where they continue to photosynthesize. They grow about 1mm per year

First Multicellular Organism–0.5/1.5bya • As colonial organisms became so successful living together, the individual cells eventually began to specialize and take on different roles • All cells in our bodies contain the same genetic information, but many of them have so many different roles, from secreting mucous to transmitting electro-chemical messages

Finding Clues in Fossils Scientists glean a great deal of information about life in the past from the fossil record. Fossil formation is a rare event and requires a certain set of circumstances to take place. This is why there are gaps in the fossil record. Most fossils formed are incomplete, but there are some that are complete. • Firstly, for a fossil to form, an organism must be buried almost immediately with silt, mud, ice, sap or sand. This protects the organism’s body from scavengers and decomposers. A common way this happens is if an organism dies near a body of water and is immediately covered with sediment • Secondly, the remains of the organism must be compressed by layers of rock where they must sit preserved for millions of years • The hidden layers of rock must then undergo geological change such as folding and then upthrust so that they are exposed to the surface ready to be found and then analysed by scientists

Finding Clues in Fossils Carbonised fossils are formed when dead organisms are exposed to extreme heat and pressure. Chemical changes occur where all the minerals in the organisms body, except carbon, are converted to gas. This type of fossil is generally black Types of Fossils Petrified fossils are formed when the bones, shells and other had parts of an animal are replaced by minerals from the surrounding soil Complete and unaltered fossil cast in amber

Finding Clues in Fossils Aging Fossils Radioactive dating Comparing strata • When a plant is living it takes in radioactive carbon 14 (14C) from the atmosphere along with stable 12C • Animals that eat the plant also take up the 14C. When the organism dies it no longer takes up 14C • 14C slowly breaks down, and after 5,700 years half of it has gone completely (this is its half life) • Scientists look at how much 14C is present in a fossil to date the age of the fossil • Scientists use Uranium (which has a half life of 4500 million years) to date rock embedded with fossils • Scientists compare the layers fossils are found in to find the relative ages of other fossils • They must know the age of one layer and they can then compare layers above or below the known layers

Finding Clues in Fossils Nanobes • As recent as 1999, NASA scientists have analysed fossils found in meteorites that came from Mars • These fossils were also analysed by an Australian scientist, Dr. Philipa Unwins, at the University of Queensland. She found evidence of fossilized organisms that came from ultra microscopic organisms • She called these organisms Nanobes

The End • Scientist have their theories and experimental data that suggests support for these theories • At the end of the day we must keep in mind that they are theories, and that perhaps we will never have the true answers to questions about the formation of the universe and life • We must also respect the beliefs of various cultures around the world concerning the beginning of life on earth; from the aborigines and their dream time to the American Indian!

The History of Life on Earth