- Explain how scientists learn about the history of life on Earth.
- Describe how and when planet Earth formed.
- Outline how the first organic molecules arose.
- Describe the characteristics of the first cells.
- Explain how eukaryotes are thought to have evolved.
- absolute dating
- fossil record
- geologic time scale
- Last Universal Common Ancestor (LUCA)
- molecular clock
- relative dating
- RNA world hypothesis
Earth formed 4.6 billion years ago, and life first appeared about 4 billion years ago. The first life forms were microscopic, single-celled organisms. From these simple beginnings, evolution gradually produced the vast complexity and diversity of life today.
The evolution of life on Earth wasn’t always smooth and steady—far from it. Living things had to cope with some astounding changes. Giant meteorites struck Earth’s surface. Continents drifted and shifted. Ice ages buried the planet in snow and ice for millions of years at a time. At least five times, many, if not most, of Earth’s living things went extinct. Extinction occurs when a species completely dies out and no members of the species remain. But life on Earth was persistent. Each time, it came back more numerous and diverse than before.
Earth in a Day
It’s hard to grasp the vast amounts of time since Earth formed and life first appeared on its surface. It may help to think of Earth’s history as a 24-hour day, as shown in Figure below. Humans would have appeared only during the last minute of that day. If we are such newcomers on planet Earth, how do we know about the vast period of time that went before us? How have we learned about the distant past?
History of Earth in a Day. In this model of Earth’s history, the planet formed at midnight. What time was it when the first prokaryotes evolved?
Learning About the Past
Much of what we know about the history of life on Earth is based on the fossil record. Detailed knowledge of modern organisms also helps us understand how life evolved.
The Fossil Record
Fossils are the preserved remains or traces of organisms that lived in the past. The soft parts of organisms almost always decompose quickly after death. On occasion, the hard parts—mainly bones, teeth, or shells—remain long enough to mineralize and form fossils. An example of a complete fossil skeleton is shown in Figure below. The fossil record is the record of life that unfolded over four billion years and pieced back together through the analysis of fossils.
Extinct Lion Fossil. This fossilized skeleton represents an extinct lion species. It is rare for fossils to be so complete and well preserved as this one.
To be preserved as fossils, remains must be covered quickly by sediments or preserved in some other way. For example, they may be frozen in glaciers or trapped in tree resin, like the frog in Figure below. Sometimes traces of organisms—such as footprints or burrows—are preserved (see the fossil footprints in Figure below). The conditions required for fossils to form rarely occur. Therefore, the chance of an organism being preserved as a fossil is very low. You can watch a video at the following link to see in more detail how fossils form: http://www.youtube.com/watch?v=A5i5Qrp6sJU.
The photo on the left shows an ancient frog trapped in hardened tree resin, or amber. The photo on the right shows the fossil footprints of a dinosaur.
In order for fossils to “tell” us the story of life, they must be dated. Then they can help scientists reconstruct how life changed over time. Fossils can be dated in two different ways: relative dating and absolute dating. Both are described below. You can also learn more about dating methods in the video at this link: http://www.youtube.com/watch?v=jM7vZ-9bBc0.
Relative dating determines which of two fossils is older or younger than the other, but not their age in years. Relative dating is based on the positions of fossils in rock layers. Lower layers were laid down earlier, so they are assumed to contain older fossils. This is illustrated in Figure below.
Absolute dating determines about how long ago a fossil organism lived. This gives the fossil an approximate age in years. Absolute dating is often based on the amount of carbon-14 or other radioactive element that remains in a fossil. You can learn more about carbon-14 dating by watching the animation at this link: http://www.absorblearning.com/media/attachment.action?quick=bo&att=832.
Relative Dating Using Rock Layers. Relative dating establishes which of two fossils is older than the other. It is based on the rock layers in which the fossils formed.
Evidence from the fossil record can be combined with data from molecular clocks. A molecular clock uses DNA sequences (or the proteins they encode) to estimate how long it has been since related species diverged from a common ancestor. Molecular clocks are based on the assumption that mutations accumulate through time at a steady average rate for a given region of DNA. Species that have accumulated greater differences in their DNA sequences are assumed to have diverged from their common ancestor in the more distant past. Molecular clocks based on different regions of DNA may be used together for more accuracy.
Consider the example in Table below. The table shows how similar the DNA of several animal species is to human DNA. Based on these data, which organism do you think shared the most recent common ancestor with humans?
Similarity with Human DNA (percent)
Geologic Time Scale
Another tool for understanding the history of Earth and its life is the geologic time scale, shown in Figure below. The geologic time scale divides Earth’s history into divisions (such as eons, eras, and periods) that are based on major changes in geology, climate, and the evolution of life. It organizes Earth’s history and the evolution of life on the basis of important events instead of time alone. It also allows more focus to be placed on recent events, about which we know the most.
Geologic Time Scale. The geologic time scale divides Earth’s history into units that reflect major changes in Earth and its life forms. During which eon did Earth form? What is the present era?
How Earth Formed: We Are Made of Stardust!
We’ll start the story of life at the very beginning, when Earth and the rest of the solar system first formed. The solar system began as a rotating cloud of stardust. Then, a nearby star exploded and sent a shock wave through the dust cloud, increasing its rate of spin. As a result, most of the mass became concentrated in the middle of the disk, forming the sun. Smaller concentrations of mass rotating around the center formed the planets, including Earth. You can watch a video showing how Earth formed at this link: http://www.youtube.com/watch?v=-x8-KMR0nx8.
At first, Earth was molten and lacked an atmosphere and oceans. Gradually, the planet cooled and formed a solid crust. As the planet continued to cool, volcanoes released gases, which eventually formed an atmosphere. The early atmosphere contained ammonia, methane, water vapor, and carbon dioxide but only a trace of oxygen. As the atmosphere became denser, clouds formed and rain fell. Water from rain (and perhaps also from comets and asteroids that stuck Earth) eventually formed the oceans. The ancient atmosphere and oceans represented by the picture in Figure below would be toxic to today’s life, but they set the stage for life to begin.
Ancient Earth. This is how ancient Earth may have looked after its atmosphere and oceans formed.
The First Organic Molecules
All living things consist of organic molecules. Therefore, it is likely that organic molecules evolved before cells, perhaps as long as 4 billion years ago. How did these building blocks of life first form?
Scientists think that lightning sparked chemical reactions in Earth’s early atmosphere. They hypothesize that this created a “soup” of organic molecules from inorganic chemicals. In 1953, scientists Stanley Miller and Harold Urey used their imaginations to test this hypothesis. They created a simulation experiment to see if organic molecules could arise in this way (see Figure below). They used a mixture of gases to represent Earth’s early atmosphere. Then, they passed sparks through the gases to represent lightning. Within a week, several simple organic molecules had formed. You can watch a dramatization of Miller and Urey’s experiment at this link: http://www.youtube.com/watch?v=j9ZRHoawyOg.
Miller and Urey’s Experiment. Miller and Urey demonstrated that organic molecules could form under simulated conditions on early Earth. What assumptions were their simulation based upon?
Recently, the findings of Miller and Urey have come into question due to discrepancies in the composition of the early atmosphere, allowing a number of other ideas to surface on the formation of the first organic molecules. One idea states that the active volcanoes on early Earth gave the necessary materials for life. Despite the simplified account discussed above, the problem of the origin of the first organic compounds remains. Despite tremendous advances in biochemical analysis, answers to the problem remain. But whatever process did result in the first organic molecules, it was probably a spontaneous process, with elements coming together randomly to form small compounds, and small compounds reacting with other elements and other small compounds to make larger compounds. So, which organic molecule did come first?
Which Organic Molecule Came First?
Living things need organic molecules to store genetic information and to carry out the chemical work of cells. Modern organisms use DNA to store genetic information and proteins to catalyze chemical reactions. So, did DNA or proteins evolve first? This is like asking whether the chicken or the egg came first. DNA encodes proteins and proteins are needed to make DNA, so each type of organic molecule needs the other for its own existence. How could either of these two molecules have evolved before the other? Did some other organic molecule evolve first, instead of DNA or proteins?
RNA World Hypothesis
Some scientists speculate that RNA may have been the first organic molecule to evolve. In fact, they think that early life was based solely on RNA and that DNA and proteins evolved later. This is called the RNA world hypothesis. Why RNA? It can encode genetic instructions (like DNA), and some RNAs can carry out chemical reactions (like proteins). Therefore, it solves the chicken-and-egg problem of which of these two molecules came first. Other evidence also suggests that RNA may be the most ancient of the organic molecules. You can learn more about the RNA world hypothesis and the evidence for it at this link: http://www.youtube.com/watch?v=sAkgb3yNgqg.
The First Cells
How organic molecules such as RNA developed into cells is not known for certain. Scientists speculate that lipid membranes grew around the organic molecules. The membranes prevented the molecules from reacting with other molecules, so they did not form new compounds. In this way, the organic molecules persisted, and the first cells may have formed. Figure below shows a model of the hypothetical first cell.
Hypothetical First Cell. The earliest cells may have consisted of little more than RNA inside a lipid membrane.
No doubt there were many early cells of this type. However, scientists think that only one early cell (or group of cells) eventually gave rise to all subsequent life on Earth. That one cell is called the Last Universal Common Ancestor (LUCA). It probably existed around 3.5 billion years ago. LUCA was one of the earliest prokaryotic cells. It would have lacked a nucleus and other membrane-bound organelles. To learn more about LUCA and universal common descent, you can watch the video at the following link: http://www.youtube.com/watch?v=G0UGpcea8Zg.
Photosynthesis and Cellular Respiration
The earliest cells were probably heterotrophs. Most likely they got their energy from other molecules in the organic “soup.” However, by about 3 billion years ago, a new way of obtaining energy evolved. This new way was photosynthesis. Through photosynthesis, organisms could use sunlight to make food from carbon dioxide and water. These organisms were the first autotrophs. They provided food for themselves and for other organisms that began to consume them.
After photosynthesis evolved, oxygen started to accumulate in the atmosphere. This has been dubbed the “oxygen catastrophe.” Why? Oxygen was toxic to most early cells because they had evolved in its absence. As a result, many of them died out. The few that survived evolved a new way to take advantage of the oxygen. This second major innovation was cellular respiration. It allowed cells to use oxygen to obtain more energy from organic molecules.
Evolution of Eukaryotes
The first eukaryotic cells probably evolved about 2 billion years ago. This is explained by endosymbiotic theory. As shown in Figure below, endosymbiosis came about when large cells engulfed small cells. The small cells were not digested by the large cells. Instead, they lived within the large cells and evolved into cell organelles.
From Independent Cell to Organelle. The endosymbiotic theory explains how eukaryotic cells evolved.
The large and small cells formed a symbiotic relationship in which both cells benefited. Some of the small cells were able to break down the large cell’s wastes for energy. They supplied energy not only to themselves but also to the large cell. They became the mitochondria of eukaryotic cells. Other small cells were able to use sunlight to make food. They shared the food with the large cell. They became the chloroplasts of eukaryotic cells.
With their specialized organelles, eukaryotic cells were powerful and efficient. They would go on to evolve additional major adaptations. These adaptations include sexual reproduction, cell specialization, and multicellularity. Eventually, eukaryotic cells would evolve into the animals, plants, and fungi we know today.
Arsenic in Place of Phosphorus - New Biochemicals for Life?
In late 2010, NASA scientists proposed the notion that the elements essential for life - carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur - may have additional members. Scientists have trained a bacterium to eat and grow on a diet of arsenic, in place of phosphorus. Phosphorus chains form the backbone of DNA, and ATP, with three phosphates, is the principal molecule in which energy is stored in the cell. Arsenic is directly under phosphorus in the Periodic Table, so the two elements have similar chemical bonding properties. This finding raises the possibility that organisms could exist on Earth or elsewhere in the universe using biochemicals not currently known to exist. These results expand the notion of what life could be and where it could be. It could be possible that life on other planets may have formed using biochemicals with elements different from the elements used in life on Earth.
In a classic example of the scientific community questioning controversial information, in the immediate six months after the original publication in the scientific journal Nature, the scientific community has raised various technical and theoretical issues concerning this finding. And as a response, the NASA team dismisses the criticism and stands by their data and interpretations.
See http://www.nytimes.com/2010/12/03/science/03arsenic.html?pagewanted=1&_r=3 and http://science.nasa.gov/science-news/science-at-nasa/2010/02dec_monolake/ for further information on this controversial finding.
- Much of what we know about the history of life on Earth is based on the fossil record. Molecular clocks are used to estimate how long it has been since two species diverged from a common ancestor. The geologic time scale is another important tool for understanding the history of life on Earth.
- Earth formed about 4.6 billion years ago. At first, Earth was molten and lacked an atmosphere and oceans. Gradually, the atmosphere formed, followed by the oceans.
- The first organic molecules formed about 4 billion years ago. This may have happened when lightning sparked chemical reactions in Earth’s early atmosphere. RNA may have been the first organic molecule to form as well as the basis of early life.
- The first cells consisted of little more than an organic molecule such as RNA inside a lipid membrane. One cell (or group of cells), called the last universal common ancestor (LUCA), gave rise to all subsequent life on Earth. Photosynthesis evolved by 3 billion years ago and released oxygen into the atmosphere. Cellular respiration evolved after that to make use of the oxygen.
- Eukaryotic cells probably evolved about 2 billion years ago. Their evolution is explained by endosymbiotic theory. Eukaryotic cells would go on to evolve into the diversity of eukaryotes we know today.
Lesson Review Questions
1. What are fossils?
2. Describe how fossils form.
3. Give an overview of how Earth formed and how its atmosphere and oceans developed.
4. Describe Miller and Urey’s experiment. What did it demonstrate?
5. State the RNA world hypothesis.
6. What was LUCA? What were its characteristics?
7. Table below shows DNA sequence comparisons for some hypothetical species. Based on the data, describe evolutionary relationships between Species A and the other four species. Explain your answer.
DNA Similarity with Species A (%)
8. Compare and contrast relative and absolute dating.
9. Why could cellular respiration evolve only after photosynthesis had evolved?
Points to Consider
The earliest organisms lived in the ocean. Even after eukaryotes evolved, it was more than a billion years before organisms lived on land for the first time.
- What special challenges do you think organisms faced when they moved from water to land?
- How do you think they met these challenges? What adaptations might they have evolved?