Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.
Conditions on Early Earth
If you took a trip back in time to when the Earth was forming about 4.6 billion years ago, you'd find a very different planet than the one we know and love today. The Earth likely began as a mass of hot, molten material that was constantly being bombarded by meteorites. As time went on, the material of Earth sorted out into layers: the inner and outer core, mantle, and crust.
Even as things calmed down over the first billion or so years, Earth would still be very unrecognizable to us. The atmosphere on young Earth had very little oxygen and was filled with water vapor, carbon dioxide, methane, ammonia, hydrogen, and other compounds that were being pushed into the air by the many volcanic eruptions. Still even more time goes by, and we see Earth cooling down a bit, with much of that water vapor in the atmosphere becoming our oceans, rivers, lakes, and streams.
Early Life on Earth
We have fossil evidence of prokaryotic life as far back as 3.5 billion years. But these life forms were not what you are familiar with as 'life' today, and even these simple organisms developed from even simpler ones that may have been around as long as 3.9 billion years ago. Scientists have determined that there are four main stages to how living things came from non-living things on Earth.
First is the formation of small organic molecules, such as amino acids and nucleotides, which make proteins and DNA. Next, these molecules link together to form bigger molecules called polymers ('poly' means 'many'). The third step to get from non-life to life is when these polymers aggregate together and form protobionts. Similar to our cells, they are surrounded by a membrane and able to maintain a stable internal environment. And finally, protobionts evolved. Protobionts were able to replicate and pass on genetic information. These were likely the first simple 'living cells' that evolved over billions of years into the millions of complex and diverse organisms we see on Earth today.
How did this all happen? We're not really sure, but there are a few hypotheses that are accepted by scientists who study this sort of thing. The first is the Oparin Hypothesis. Proposed by Russian chemist A.I. Oparin in the 1920s, this hypothesis describes how Earth's early atmosphere was very low in oxygen and very reactive from intense lightning and UV radiation. Oparin also hypothesized that the early oceans on Earth contained a primordial soup, or organically rich solution. There were many different elements and compounds in this 'soup' that over time, and in early Earth's reactive environment, took simple compounds and made them into more complex ones.
In the 1950s, two men, Stanley Miller and Harold Urey, decided to test this hypothesis by recreating the conditions of early Earth in the lab. They designed an apparatus that simulated how this process would have worked based on the hypothesized ocean conditions, gases in the atmosphere, lighting, and radiation thought to be present. While they didn't recreate 'life' per se, they did find amino acids (the building blocks of proteins), as well as oily hydrocarbons, which are used for energy. Did life come from these small beginnings? They sure thought so!
Life Moves On
Obviously a lot has happened since these primitive beginnings of life! Luckily, the history of life on Earth has been divided into major sections called eons, of which there are three. These are, from the oldest to the most recent, the Archean, Proterozoic, and Phanerozoic.
The Archean Eon spans the time from the beginning of Earth until about 2.5 billion years ago, which is when we see the first fossils of eukaryotic cells. Photosynthetic prokaryotic organisms likely developed during this time period, which helped produce a large amount of oxygen in the atmosphere, a key player in life on Earth.
From this point until about 542 million years ago is the Proterozoic Eon, and together with the Archean Eon (about 4 billion years total) we call this time Precambrian. During the Proterozoic Eon we see lots of changes on Earth. Simple cells evolve into more complex ones, multi-cellular organisms develop, and even some soft-bodied invertebrates such as worms, sponges, and jellies emerge during this time period.
The Phanerozoic may be the shortest eon, only spanning the last 550 million years or so, but it's also the one we have the most information about. There are three eras within this eon known as the Paleozoic, Mesozoic, and Cenozoic. Each of these eras is also divided into periods, which are further divided into epochs, but we won't go into that much detail here.
The first period of the Paleozoic Era is called the Cambrian Period, and is also where we find the Cambrian Explosion. During this period, Earth experiences a surge of diversity in life, especially animals. Throughout the rest of the Paleozoic Era we also see the emergence of bony fishes, tetrapods, reptiles, amphibians, and insects, as well as seed and vascular plants, marine algae, and a great diversification of fungi.
The Mesozoic Era (think 'meso' for 'middle') is when we see the emergence of cone-bearing and flowering plants, but most importantly this is the time of the dinosaurs! Unfortunately for them though, they also go extinct at the end of this era as well.
Finally, we have the Cenozoic Era, which is when we see an explosion in the diversity of mammals, birds, and pollinating insects. We also see primates emerge, as well as our earliest human ancestors. And to think, all this diversity in just half a billion years. That's pretty amazing!
When we look at the history of life on Earth, we embark on an incredible journey. We start 4.6 billion years ago, when Earth was a hot, molten mass. As it cooled, it formed layers, the atmosphere changed dramatically, and the oceans formed on the surface. Though we are not sure exactly how or when life came about during these early stages, scientists are in agreement that it likely developed from small, organic molecules such as amino acids and nucleotides. These are the building blocks for proteins and DNA, which are the building blocks for living organisms. These small molecules joined together as polymers, which then aggregated together into protobionts. These then evolved to pass on genetic information, which eventually led to the wide diversity of organisms we see today. Oparin originally proposed this hypothesis in the 1920s, and Miller and Urey actually tested it in the 1950s.
But the beginning of life was, well, just the beginning! Until about 2.5 billion years ago, Earth was still figuring itself out. But once photosynthetic prokaryotes started putting oxygen into Earth's atmosphere, life became more abundant and diverse. This first part of Earth's history falls in the Archean Eon, followed by the Proterozoic Eon, which is where we see the first eukaryotic fossils and even some soft-bodied invertebrates.
The Cambrian Explosion marks not only the first period of the Phanerozoic Eon, but also when life on Earth 'explodes' with diversity. Though this eon only covers about 550 million years, we see the emergence of fishes, mammals, flowering plants, insects, reptiles, dinosaurs, and even humans.
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