Context: Its hard to grasp the vast amounts of time since Earth formed and life first appeared. It may help to think of Earths history as a 24-hour day. Figure 7.17 shows the history of Earth in a day. In this model, the planet forms at midnight. The first prokaryotes evolve around 3:00 am. Eukaryotes evolve at about 1:00 pm. Animals dont evolve until almost 8:00 pm. Humans appear only in the last minute of the day. Relating these major events in Earths history to a 24-hour day helps to put them in perspective. Another tool for understanding the history of Earth and its life is the geologic time scale. You can see this time scale in Figure 7.18. It divides Earths history into eons, eras, and periods. These divisions are based on major changes in geology, climate, and the evolution of life. The geologic time scale organizes Earths history on the basis of important events instead of time alone. It also puts more focus on recent events, about which we know the most. The Precambrian Supereon is the first major division of Earths history (see Figure 7.18). It covers the time from Earths formation 4.6 billion years ago to 544 million years ago. To see how life evolved during the Precambrian and beyond, watch this wonderful video. Its a good introduction to the rest of the lesson. MEDIA Click image to the left or use the URL below. URL: When Earth first formed, it was a fiery hot, barren ball. It had no oceans or atmosphere. Rivers of melted rock flowed over its surface. Gradually, the planet cooled and formed a solid crust. Gases from volcanoes formed an atmosphere, although it contained only a trace of oxygen. As the planet continued to cool, clouds formed and rain fell. Rainwater helped form oceans. The ancient atmosphere and oceans would be toxic to modern life, but they set the stage for life to begin. All living things consist of organic molecules. Many scientists think that organic molecules evolved before cells, perhaps as early as 4 billion years ago. Its possible that lightning sparked chemical reactions in Earths early atmosphere. This could have created a soup of organic molecules from inorganic chemicals. Some scientists think that RNA was the first organic molecule to evolve. RNA can not only encode genetic instructions. Some RNA molecules can carry out chemical reactions. All living things are made of one or more cells. How the first cells evolved is not known for certain. Scientists speculate that lipid membranes grew around RNA molecules. The earliest cells may have consisted of little more than RNA inside a lipid membrane. You can see a model of such a cell in Figure 7.19. The first cells probably evolved between 3.8 and 4 billion years ago. Scientists think that one cell, called the Last Universal Common Ancestor (LUCA), gave rise to all of the following life on Earth. LUCA may have existed around 3.5 billion years ago. The earliest cells were heterotrophs. They were unable to make food. Instead, they got energy by "eating" organic molecules in the soup around them. The earliest cells were also prokaryotes. They lacked a nucleus and other organelles. Gradually, these and other traits evolved. Photosynthesis evolved about 3 billion years ago. After that, certain cells could use sunlight to make food. These were the first autotrophs. They made food for themselves and other cells. They also added oxygen to the atmosphere. The oxygen was a waste product of photosynthesis. Oxygen was toxic to many cells. They had evolved in its absence. Many of them died out. The few that survived evolved a new way to use oxygen. They used it to get energy from food. This is the process of cellular respiration. The first eukaryotic cells probably evolved about 2 billion years ago. Thats when cells evolved organelles and a nucleus. Figure 7.20 shows one theory about the origin of organelles. According to this theory, a large cell engulfed small cells. The small cells took on special roles that helped the large cell function. In return, the small cells got nutrients from the large cell. Eventually, the large and small cells could 
 Question: If Earths history is represented by a 24-hour day, then life first appears at about
Output: 3:00 am