Chapter 18: Life On Earth

More profound than the question "Are we alone?" may be the question "How did we get here?". Our natural curiosity as humans leads us to deep philosophical questions, and science is a tool we use to reveal the answers. However, when it comes to piecing together the mystery of how life arose on Earth, we are far from a complete answer. Investigating the origin of life on Earth is not a trivial endeavor. The biological and geological histories of Earth are intimately entwined, not separate tapestries waiting to be unraveled. Uncovering the story of the origin of life, therefore, requires the collaboration of scientists from many different disciplines.

The origin of life on Earth represents the very beginning of biology on our planet. To better understand the events leading to the origin of life, we have to examine more than just biology. We must carefully consider the environmental conditions of early Earth, the source of "life essentials" such as organic molecules and water, and the mechanisms that would have resulted in early cellular life. 

An artist's depiction of a collision between two planetary bodies. Such an impact between the Earth and a Mars-sized object likely formed the Moon and the large objects that hit the Earth around this time were part of a random process.

Our Earth formed at the same time as all the other planets in the Solar System, about 4.5 billion years ago. The first undisputed evidence for life on Earth appears in the geologic record approximately 3.8 billion years ago. What was happening during those first few hundred million years? At the time of Earth's formation, the Solar System was not a very nice place. Earth was constantly bombarded by planetesimals. Oceans had not yet formed on the planet and the crust was still molten. Until bombardment by meteors, comets, and other space debris slowed, Earth was not habitable. During these first few hundred million years, Earth was changing significantly; the crust cooled and solidified, the oceans formed, and the conditions on Earth began to stabilize. Once these things happened, it is thought that life almost immediately began.

Scientists are unsure exactly what Earth was like during those first few hundred million years. There are no rocks from this time period because of plate tectonics and the dynamic nature of the Earth's surface. The oldest rocks identified date to 3.9 billion years ago. However, there are crystals of zirconium silicate — zircons — that are 4.4 billion years old.  In addition to the lack of information in the geologic record, there is also no way to know what the atmosphere was like at that time, although it is certain that Earth was not oxygen-rich until a few billion years ago.  Currently, most scientists agree that our early atmosphere was "mildly reducing" which means it was dominated by carbon dioxide (CO₂) and nitrogen (N₂) with small amounts of carbon monoxide (CO), hydrogen (H₂), and reduced sulfur gases. Whatever the actual composition turns out to be, it is important to note that oxygen was not a key player. Due to the lack of oxygen, some of the first forms of life on Earth were most likely anaerobic: organisms that do not require oxygen to live. It has also been proposed that these first life forms were probably photosynthetic, therefore releasing oxygen into the atmosphere as a byproduct. The atmosphere would have remained reducing until enough oxygen was released by living organisms to raise the amount of oxygen, and eventually ozone, to appreciable levels.

Before life could begin on Earth, the components for life had to be present. At the very minimum, the first form of life needed water and organic molecules to create the cells that we readily associate with living organisms. Again, scientists are unsure how Earth got its supply of water and organic material.  We have already mentioned that Earth's oceans formed relatively early on. The isotopic composition of zircons suggests that liquid water was present on Earth as early as 4.4 billion years ago. Where did all that water come from? Earth is one of four terrestrial planets in our Solar System. Terrestrial planets are characterized by their rocky compositions and lack of volatiles. including water. Water is relatively abundant in the Solar System, but it is concentrated in the outer planetary bodies. As a volcanically active planet, it is likely that water incorporated into Earth during accretion could have been released into the atmosphere through volcanic outgassing. However, it is doubtful that sufficient water could have resulted from accretion and outgassing alone. Some scientists propose that collisions of comets with Earth during the period of heavy bombardment may have delivered water. Based on what we currently know about comet composition, this mechanism of delivering water may have only contributed 10% of Earth's water. More recent studies of carbonaceous and ordinary chondrites — some of the oldest materials in the Solar System — have caused other scientists to suggest that meteorites brought water to Earth. Regardless of exactly how it is probable that all three of these methods contributed in some way to Earth's water reservoir.

small timeline showing different events in the evolution of life

What about complex organic molecules, like amino acids? Stated simply, organic molecules necessary for life as we know it either originated on Earth, outside of Earth, or a combination of those two scenarios. Again, it is difficult to provide a concrete response since we have limited knowledge of the early Earth. However, scientists have worked to simulate early Earth conditions in the laboratory setting. The first of these scientists were Harold Urey and Stanley Miller in the early 1950s. Miller was a bright young graduate student of Urey. Together they created an analog of Earth's early atmosphere. To this atmosphere, they added energy in the form of a spark, meant to represent lightning on early Earth. What they discovered from this simple experiment was that a variety of complex organic molecules formed readily in these conditions. Although it did not create all of the 20 amino acids used by life forms today, it was the first evidence that organic molecules could have been a natural chemical result on early Earth. More recently, scientists have been exploring the nature of matter beyond Earth. It turns out that organic molecules are not as rare as was once thought.  Indeed, some scientists are suggesting that Earth received its supply of complex molecules from comet and meteorite collisions with Earth.

Once the conditions on Earth were conducive, it is thought that life developed quickly. It is important to understand that "quickly" in this case should be thought of within the context of geologic time scales and can represent several thousand to several million years. There is no doubt that the conditions and materials necessary for the origin of life were all present. However, there is great debate about how and where life first formed on Earth. Did the first form of life have DNA, or was some other molecule used to store information? How were preliminary cells structured?  How were the first proteins synthesized? Many scientists agree that DNA might have been too complicated to be the first information storage molecule. Instead, RNA or some other molecule may have filled the role until cells evolved to utilize DNA. No one knows, but there has been a lot of exciting work examining these first molecules. After examining evolutionary relationships between organisms, there have also been proposals about where life formed. Some scientists feel that the first life forms on Earth originated in hot environments, like around hydrothermal vents or in hot springs. On the other hand, some biochemists studying the stability of DNA and cell membranes argue against this idea. High temperatures would not have been conducive to cell membrane formation and would have contributed to the degradation of organic molecules like DNA and RNA. 

Comparison of a single-stranded RNA and a double-stranded DNA with their corresponding nucleobases.

There are still many missing pieces to the puzzle of "how we got here". An exciting avenue in the past decade is the use of lab experiments to simulate steps along the road from simple chemicals to the first cell. The work of Jack Szostak and his group at Harvard is particularly notable. Szostak has shown that simple fats dissolved in water can lead to vesicles or proto-cells. They are much smaller than modern cells but they act to contain and concentrate chemicals and lead to greater complexity of those chemical ingredients over time. More excitingly, Szostak has shown that if clay particles are in the proto-cells, small fragments of RNA can be built naturally — the periodic lattice of the clay mineral acts as a surface for "templating" the small pieces of RNA. Fragments as large as a thousand atoms have been seen in these experiments. The work continues, and one thing is for certain, the more we understand the origins of life on our own planet, the better prepared we will be at predicting where to look for life elsewhere.