Middle School Life Science: Evolution - History of Life on Earth — Flashcards

Many scientists think that organic molecules evolved before cells, perhaps as early as 4 billion years ago. It's possible that lightning sparked chemical reactions in Earth's 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 encode genetic instructions, and some RNA molecules can also carry out chemical reactions.    
All living things are made of one or more cells. How the first cells evolved is not known for certain. The earliest cells may have consisted of little more than RNA inside a lipid membrane. The first cells probably evolved between  .8 and 4 billion years ago. These 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 because 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 by cellular respiration.    
- The first eukaryotic cells probably evolved about 2 billion years ago. That's when cells evolved organelles and a nucleus. According to the most widely accepted theory, this occurred when 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 no longer live apart.    
- With their specialized organelles, eukaryotic cells were powerful and efficient. They would go on to evolve sexual reproduction. They would also evolve into multicellular organisms. The first multicellular organisms evolved about 1 billion years ago.