What did early eukaryotic life look like?
Some looked like this. This is a fossil of an ammonite. Ammonites are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.
Multicellular Life: Setting the Stage
Nearly 80% of Earth’s history passed before multicellular life evolved. Up until then, all organisms existed as single cells. Why did multicellular organisms evolve? What led up to this major step in the evolution of life? To put the evolution of multicellularity in context, let’s return to what was happening on planet Earth during this part of its history.
The Late Precambrian
The late Precambrian is the time from about 2 billion to half a billion years ago. During this long span of time, Earth experienced many dramatic geologic and climatic changes.
- Continents drifted. They collided to form a gigantic supercontinent and then broke up again and moved apart. Continental drift changed climates worldwide and caused intense volcanic activity. To see an animation of continental drift, go to this link: http://www.ucmp.berkeley.edu/geology/anim1.html.
- Carbon dioxide levels in the atmosphere rose and fell. This was due to volcanic activity and other factors. When the levels were high, they created a greenhouse effect. More heat was trapped on Earth’s surface, and the climate became warmer. When the levels were low, less heat was trapped and the planet cooled. Several times, cooling was severe enough to plunge Earth into an ice age. One ice age was so cold that snow and ice completely covered the planet. Earth during this ice age has been called snowball Earth (see Figure below).
Snowball Earth. During the late Precambrian, Earth grew so cold that it was covered with snow and ice. Earth during this ice age has been called snowball Earth.
Life During the Late Precambrian
The dramatic changes of the late Precambrian had a major impact on Earth’s life forms. Living things that could not adapt died out. They were replaced by organisms that evolved new adaptations. These adaptations included sexual reproduction, specialization of cells, and multicellularity.
- Sexual reproduction created much more variety among offspring. This increased the chances that at least some of them would survive when the environment changed. It also increased the speed at which evolution could occur.
- Some cells started to live together in colonies. In some colonies, cells started to specialize in doing different jobs. This made the cells more efficient as a colony than as individual cells.
- By 1 billion years ago, the first multicellular organisms had evolved. They may have developed from colonies of specialized cells. Their cells were so specialized they could no longer survive independently. However, together they were mighty. They formed an organism that was bigger, more efficient, and able to do much more than any single-celled organism ever could.
The Precambrian Extinction
At the close of the Precambrian 544 million years ago, a mass extinction occurred. In a mass extinction, many or even most species abruptly disappear from Earth. There have been five mass extinctions in Earth’s history. Many scientists think we are currently going through a sixth mass extinction. What caused the Precambrian mass extinction? A combination of climatic and geologic events was probably responsible. No matter what the cause, the extinction paved the way for a burst of new life, called the Cambrian explosion, during the following Paleozoic Era.
- During the late Precambrian, continents drifted, carbon dioxide levels fluctuated, and climates changed. Many organisms could not survive the changes and died out.
- Other organisms evolved important new adaptations. These include sexual reproduction, cell specialization, and multicellularity.
- The Precambrian ended with a mass extinction, which paved the way for the Cambrian explosion.
Use the time slider in this resource to answer the questions that follow.
- Two billion years ago, what was the percentage of oxygen in the atmosphere?
- When did multicellular organisms first appear?
- When did photosynthesis begin on land?
- When did sponges and fungi first evolve?
- What was the time of the Snowball Earth?
- How long did the Snowball Earth last? What caused the Earth to thaw?
1. Describe geologic and climatic changes that occurred during the late Precambrian.
2. What is a mass extinction?