The Tree of Life

Welcome to the next StemTalk! This is going to be about the tree of life. This talk is going to go over the history of Earth’s formation in a very brief manner.

Earth formed around 4.6 billion years ago, condensing from a large cloud of dust and space rock to form the first structure. Condensation occurred and repeated due to a buildup of gravity as the mass grew larger in size. Collisions of the rocks generated a lot of heat, vaporizing water, which prevented the formation of seas and oceans on Earth. For the first few million years, our planet was uninhabitable. The first atmosphere consisted of Helium and Hydrogen. The Earth looked a little like this:

As time progressed forward, Earth began to become more habitable due to changes in temperature, cooling, the formation of soil layers, and a newer atmosphere. Life came to the oceans and seas as tiny microscopic organisms. Much of early life was very simple, prokaryotic and unicellular. A certain organism, cyanobacteria, was able to use CO2, sunlight, and water to create its own energy, and release oxygen. Sounds familiar? This was one of the first photosynthetic organisms to arise. An abundance of these bacteria led to the change in Earth’s atmosphere, the event called the Oxygen boom. Our atmosphere began taking shape with 21% Oxygen, 78% Nitrogen, 0.9% Argon, and 0.1% Carbon Dioxide and other gases. Life evolved and adapted to live on land and become more complex. Eukaryotes and multicellular organisms formed. Our Earth began to become the haven of life we know today.

There are many hypotheses to how exactly organic molecules, the backbone of life, came to our planet. Some believed that organic matter came from volcanoes and some believe that it came from an extraterrestrial source.

The most famous explanation for the beginning of life begins with Oparin’s hypothesis. Oparin hypothesized that the early atmosphere was a reducing environment-meaning it donated electrons-that reduced simple molecules to make them into organic compounds. This theory was proved later in 1953 by Stanley Miller and Harold Urey. In their experiment, they set up conditions of an early Earth, applying the reducing atmosphere with Hydrogen, methane, ammonia, and water vapor. They used electrodes to send sparks to symbolize lightning, and a condenser to cool the atmosphere and cause rain to fall. The experiment was set up like this:

When they took the samples after running the system, they found various different organic molecules that were not present before. They had proven Oparin’s hypothesis. Life was formed from a reducing atmosphere.

Organic compounds were discovered to generate, but how did they form into complex polymers? Amino acid polymers, or peptides, formed when monomers reacted with hot sand spontaneously. The heat caused them to bond forming polymers. (P.S If there is any terminology you do not understand they have been talked about in previous posts :D) From this discovery rose protobionts, a combination of abiotically produced molecules enveloped by a membrane structure. They give off properties associated with life such as metabolism and reproduction. They were proved to be able to form spontaneously.

Along with proteins, the first nucleic acids was estimated to be RNA. RNA was found to carry many metabolic functions, and these enzyme functions of RNA are called ribozymes. RNA can take on 3D shapes to become more stable, showing it can also maintain homeostasis and can replicate.

A key tool we use to find out more about our planet’s past is the fossil record. The fossil record is our collection of fossils found in the ground, used to determine times that the organism existed and functions the organism could carry out. They also help determine phylogeny. (Which we will have a talk on later :D) Fossil records can show when a species dies out, or another species begins which also hints at the many mass extinctions that have occurred on Earth. Fossils are dated using multiple techniques like radiometric dating. This is based on the decay of radioactive isotopes found on the fossils, and comparing it to the fixed rate of decay of each isotope. The half-life of an isotope is the number of years it takes for half the original amount to decay. Another technique called magnetic reversals measures the magnetism of rocks with a magnetometer. This works because Earth’s poles have been determined to reverse repeatedly, leaving their magnetic reversals on the rocks. Patterns of reversals of a rock in a location can be compared to a similar pattern in another location.

A geologic record was established dividing Earth’s history into three eons. Archaen and Proterozoic were the first two eons lasting for 4 billion years in total. The Phanerozoic eon is around half a billion years and divided into three eras: Paleozoic, Mesozoic, and Cenozoic.

Using fossil dating techniques, the oldest known fossils were from 35 billion years ago called stromatolites. These are structures of rock consisting of many layers of sediment and bacteria. The protobionts were heterotrophic and needed to obtain their molecules from the primitive soup. Eventually, autotrophs replaced protobionts such as cyanobacteria, and this led to the Oxygen boom. Oxygen reacted with iron and caused iron oxide to precipitate, accumulating as sediments, and then further compressed into rock. This was how stromatolites were formed.

Another controversial topic is how eukaryotes came to be. The oldest eukaryotes were dated at 2.1 billion years ago. The main theory of how eukaryotes arose from prokaryotes is endosymbiosis. Endosymbiosis states that prokaryotes entered other larger host cells as undigested prey or parasites, combining to work out a mutually beneficial relationship. This is how mitochondria and chloroplasts were said to enter cells since the organelles each have their own nucleic acids. This means that at one point the two organelles were prokaryotic organisms. Serial endosymbiosis states that mitochondria evolved before chloroplasts.

This shared DNA between the organelles and the host cell led to another process called genetic annealing. This is when horizontal gene transfers occur between multiple organisms’ genomes, causing eukaryotic organisms.

As eukaryotes evolved, multicellular organisms began to appear. The common ancestor of multicellular eukaryotes is dated to be 1.5 billion years ago. The snowball Earth hypotheses state that glaciers covered the Proterozoic period, causing the seas to be solid ice. This caused most life to be confined to areas such as deep-sea vents to attain heat. This is an explanation for why multicellular organisms were not in abundance.

First multicellular organisms lived in colonies, or groups of replicating cells, which specialized over time to their environments to come as one and become multicellular.

Geologists also discovered that most major phyla of animals showed up from the fossil record in the first years of the Cambrian. They deemed the event, “The Cambrian Explosion,” where fossils of two animal phyla, Cnidaria and Porifera, appeared.

Sea Anemone (relative of Cnidaria)

Cyanobacteria, populating damp surface water bodies such as swamps, evolved into plants that grew on land. Arthropods and vertebrates began to appear. Tetrapods, four-limbed vertebrates, began to live on land.

As life appeared on land, fossil records of these organisms were spotted in different locations. This led to the discovery of continental drift, the gradual movement of the tectonic plates of the continental and oceanic crusts over Earth’s surface thanks to convection currents in the mantle. (We will discuss this further in an Environmental Science post) Pangaea, meaning all land, was the supercontinent that existed at the end of the Paleozoic, separating over time due to continental drift and ultimately forming the setting of Earth’s land and oceans as it looks today.


The tree of life, began at a lifeless Earth, to prokaryotic unicellular organisms, to eukaryotes, to multicellular life. The taxonomic systems were created by Carolus Linnaeus to organize and classify all organisms on our planet that we know of. It previously had two kingdoms: Plantae and Animalia. In 1969, Robert H. Whittaker fought for a five kingdom system: Plantae, Animalia, Fungi, Protista, and Monera. Not long ago, the new three-domain system was proposed. It contains the domains bacteria, archaea, and eukarya as superkingdoms called domains. Under each domain exists multiple kingdoms. For example, under Eukarya exists Animalia and Plantae. The key to understanding this taxonomic system is that it is not without error. It is an attempt of humans to order life, and this is always changing, shifting, and could be revoked from new discoveries through fossil records.

Thank you! I hope you learned a lot about the history of life on Earth in this StemTalk. Make sure to comment if you have any questions and stay tuned for more talks on interesting informative topics. 😀

-Written By: Neil D. 2/13/2019


Biology 7th Edition Campbell Reece –

Neil A. Campbell
Jane B. Reece

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