The Beginning of Life on Earth
By Patti Hutchison
When did life begin on earth? We know it did not begin when the earth was formed 4.6 billion
years ago. The earth was constantly being struck by meteorites at that time. Also, there was no
oxygen in the air. The first fossils of life were formed about 3.5 billion years ago. This makes
scientists believe the first life began between 3.9 and 3.5 billion years ago.
1
Scientists believe that a kind of atmospheric "soup" of hydrogen, ammonia, and methane
reacted when lightning struck them. They formed the building blocks of life. These building blocks
are called amino acids. They form proteins. Proteins are the basic substances of living things.
2
Proteins were the first step. Other substances needed are DNA and RNA. These substances
allow living things to reproduce. This is a basic characteristic of living things.
3
DNA carries the instructions for cell function. RNA can act as an enzyme that also helps cells
function.
4
Some scientists believe that life began deep under the sea near hydrothermal vents. Here,
ocean water seeps through the cracks in the ocean floor. The water is heated by the magma. It
rises through the ocean. All of the energy and nutrients needed for life are found near these
hydrothermal vents.
5
Some scientists believe that near these vents of hot water, amino acids joined together on clay
minerals to form proteins. Others believe that this is not possible in this hot environment. Life is not
being formed at these places today. Amino acids are quickly eaten up by organisms that live near
the vents. However, during the early history of the earth, there were no living organisms to eat the
amino acids that were produced.
6
Another theory says that fats in the early ocean "globbed" together. They enclosed the amino
acids inside them. These structures formed primitive cells. There are several other theories about
how life began on earth. Scientists are not really sure, even
billions of years later!
7
The first fossils are of prokaryotes. These organisms were
one-celled. The cells did not have a nucleus.
8
Scientists believe that the next form of life were
eukaryotes. These are larger than prokaryotes. They are
composed of cells that have a nucleus. The oldest eukaryote
fossils are 2.1 billion years old.
9
Around 700 million years ago, the earth had a major ice
age. Glaciers advanced almost to the equator. Many
eukaryotes became extinct. After the ice receded, many-celled organisms appeared in the fossil
record.
10
Did early life form from alien invasions? Did it form in the oceans? Scientists are not yet sure.
The fossil record tells us what kind of life forms existed on the early earth. They also show us how
these organisms changed over time.
11
Name___________________________________
Mr. Hobbs
Date____________________
“The Beginning of Life on Earth” HW
1. Scientists believe life began:
a. 2.1 billion years ago
b. Between 3.9 and 3.5 billion years ago
c. Between 3.9 and 3.5 years ago
3. DNA and RNA help organisms:
2. Amino acids formed in:
a. Proteins
b. Hydrothermal vents
c. Soup
4. What are prokaryotes?
a. Become extinct
b. Digest food
c. Reproduce
5. When did many eukaryotes become extinct? 6. All scientists agree about how life first
formed on earth.
True
False
7. According to the information in paragraph 8, explain TWO (2) reasons that YOU are different
than the organisms that are in the first fossils. Support each of your reasons with specific evidence
from the article.
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The Beginning of Life on Earth - Answer
Key
1
Between 3.9 and 3.5 billion years ago
2
Proteins
3
Reproduce
4 one-celled organisms that did not have a nucleus
5 during an ice age 700 million years ago.
6
False