How Did Life Begin?

Life on Earth is thought to have started as simple, one celled organisms approximately 4 billion years ago. For about 3 billion years, only single celled microorganisms existed, but once cells with a true nucleus developed about a billion years ago, multicellular organisms evolved.

The Beginning of Life

Artist's Rendition of the Big Bang

There is nobody who knows exactly how the universe began, how the Earth came about, or how life on Earth started. In fact, it is likely that we will never know for sure. This is the important part about science: there are no definite answers to questions, but there are answers that are commonly accepted.

If you drop a pen and it falls to the ground, a scientist would say that it fell due to the force of gravity. However, gravity is just one idea, an idea that happens to explain our experience of the universe around us very well. Consider this: Brandon and Jasmine, scientists from East Cleveland, come around and discover evidence that there is a mysterious glue which holds everything in the universe together, and then they find that it is the glue which caused the pen to fall to the ground. Brandon and Jasmine could take this evidence, write up a lab report, send it to other scientists, and then see if other scientists can make the same thing happen. If enough other scientists agree, the theories and laws actually change! In other words, nothing is set in stone.

The Earth, from space

According to all of the evidence, scientists theorize that the following is true: the universe began 13.7 billion years ago, the Earth was formed about 4.6 billion years ago, life on Earth began about 4 billion years ago with unicellular organisms, and multicellular organisms evolved about 1 billion years ago. What does all of this really mean? And how do scientists “know” this?

First of all, it means that the universe has been around for a very, very, very, very, very, VERY long time. It started with something called the Big Bang when all of the matter in the universe exploded out of a single point at a speed that is unimaginable. Scientists came up with this number by measuring something called cosmic background radiation that was given off by the Big Bang.

On Earth

The Big Bang created galaxies of stars. From there, it took about 9 billion years just for our Earth to be separated from our star, the sun, and start to cool down enough so that a few million years later, life could develop. Nobody is exactly sure how life began, but a few scientists have been able to create the building blocks of DNA and cells from the basic elements of a young Earth, light and heat. It is thought that with enough time (throw in a few million years), that a cell develops from this basic stew of ingredients.

Even though it only took about 500 million years for life to develop on Earth, it took a very long time for complex life, eukaryotes and then multicellular organisms, to evolve. Before eukaryotes, no cells had a nucleus. They were all prokaryotes. But it is thought that one cell swallowed another, and instead of killing the prokaryote, this cell started to put the prokaryote to work! The prokaryote became the nucleus of the new eukaryote cell, and from there life began to get very complicated very quickly.

Within a few million years, plants, fungi and animals all evolved and one billion years later, humans evolved from some unknown ancestor. To get from the Big Bang to the first human took such a long time that you can imagine the following scenario: you are driving from Los Angeles to New York, a distance of about 3000 miles. The Big Bang happens when you leave Los Angeles, the Earth is formed when you cross the Mississippi River, and eukaryotes evolve by the time you are just outside Philadelphia (about 1½ hours from New York). Humans? The first humans are seen in the last block (200 feet) before you cross into New York and your entire lifetime is less than the width of a hair on the ground on that block.

1. Describe the two types of cells.
2. Complete: The beginning of the universe, the ___________ happened about _______ billion years ago and life on Earth began _______ billion years ago.
3. How was Earth formed?
Put it together
4. Explain why science doesn't have firm answers to questions.
5. Create a map showing the “evolutionary trip” from Los Angeles to New York in the last paragraph of the reading.
Think about it
6. Predict what kind of evidence might make scientists think that life on Earth was in fact much older than 4 billion years.
7. What is:
a) An experimental group?
b) A control group?

Cosmic Calendar

January 1st of the one-year “Cosmic Calendar” represents the Big Bang, which scientists theorize is the beginning of cosmic time. “Today” is represented by the last possible moment on December 31st.

  1. What were ten of the important events that happened in the universe between the Big Bang and now?
  2. Individually, you will make a calendar with two sides: one for events in your life and one for events in the universe. You will use four pieces of paper attached on the short side or strips of paper provided by the teacher.
  3. Draw a line down the middle of the four strips. On the top, you will label the calendar with the dates in your life. You will start with your birth (e.g., 15 years ago) and work up to today (present day). Make sure that the space between each year is the same.
  4. On the bottom of the line, you will label the calendar with the dates of the universe. You will start with the Big Bang (13.7 billion years ago) and then work up to today, labeling every billion years. Make sure that the space between every billion years is the same! For the last billion years, label 500 million years ago (halfway), 250 million years ago (one quarter of the way) and 1 million years ago (one one-thousandth of the way!) on the timeline.
  5. For the ten important events in your life, make sure to include several that happened within the past few months, including the beginning of school and the beginning of today’s class. Make a sign or picture for each on a separate sheet of paper that you can tape to your timeline. Put each on the timeline where it occurred.
  6. For the ten important events in the universe, make a sign or picture for each on a separate sheet of paper that you can tape to your timeline. Put each on the timeline where it occurred.
  7. Share your estimates with the other students and post it up around the room.
mya (million years ago) Event
13,700 Big Bang; universe comes into being
4,600 Earth comes into being
3,500 First primitive life; single-celled prokaryotes
1,000 First eukaryotes evolve; first multicellular organisms
500 First vertebrate life
430 First plants evolve
400 First land animals evolve
200 Dinosaurs evolve
65 Dinosaur extinction
5 Hominids branch off from other primates
2 Homo habilis (the toolmaker) evolves
1.60 Homo erectus (stands upright, can speak) evolves
0.20 Homo sapiens evolves; adaptation to cooler climates with fire, housing, clothing
0.10 Homo sapiens sapiens evolves; modern humans
0.02 Last ice age peaks
Cosmic Calendar: Toilet Paper


  • One roll of toilet paper, 231 sheets or more.
  • Felt-tip marker(s) or fluid writing utensil(s), preferably several colors.
  • Clear tape for repairs.


  1. Starting at one end of a long hallway, unroll the toilet paper until you reach 230 squares.
  2. Label the events on the appropriate square of toilet paper with the event and the real date in mya.
  3. Take a look at all of the events. How long have humans been on Earth?
  4. What three things about the cosmic calendar surprise you? Why?


Sheets Event Geological time (Number of years before present) Comments
0.00 Present 0


Modern man



Neanderthal man



First use of fire



Worldwide glaciation



Homo erectus



Linking of North and South America



Oldest stone tools



Beginning of Quaternary period (end Tertiary/Neogene)






Beginning of Antarctic ice caps



Opening of Red Sea



Formation of Himalayan Mountains



Beginning of Tertiary/Neogene period (end Paleogene)



First evidence of ice at the poles



Collision of India with Asia



Early horses



Separation of Australia and Antarctica



Early primates



Opening of Norwegian Sea and Baffin Bay



Alps form



Beginning of Tertiary/Paleogene period



Beginning of Cenozoic Era


“recent life”


Cretaceous Period, Mesozoic Era end



Dinosaurs became extinct



Rocky Mountains form



Cretaceous Period begins (Jurassic ends)



Early flowering plants



Early birds and mammals



Jurassic Period begins (end Triassic)



Opening of Atlantic Ocean



Triassic Period begins



Beginning of Mesozoic Era (end Paleozoic)


“middle life”


Final assembly of Pangaea



Beginning of Permian period (end Carboniferous/Pennsylvanian)



First reptiles



Beginning of Carboniferous/Pennsylvanian period (end Mississippian)



Early trees, formation of coal deposits



Beginning of Carboniferous/Mississippian period (end Devonian)



Beginning of Devonian period (end Silurian)



Early land plants



Beginning of Silurian period (end Ordovician)



Early fish



Beginning of Ordovician period (end Cambrian)



Early shelled organisms



Beginning of Cambrian period (end of Precambrian time)


rise of multicellular animals


Beginning of Paleozoic Era


“ancient life”


Beginning of Phanerozoic Eon (end Proterozoic)


“visible life” (or 544 million years ago)


Early multicelled organisms



Breakup of early supercontinent



Formation of early supercontinent



First known animals



Beginning of Proterozoic Eon (end Archeon)


“earlier life”


Buildup of free oxygen in atmosphere



Early bacteria & algae



Oldest known Earth rocks



Beginning of Archeon Eon



Precambrian time begins



Origin of earth


Relative Dating

From Marsha Barber and Diana Scheidle Bartos


Scientists have good evidence that the earth is very old, about 4.6 billion years old. Scientific measurements such as radiometric dating use the natural radioactivity of certain elements found in rocks to help determine their age. Scientists also use direct evidence from observations of the rock layers themselves to help determine the relative age of rock layers. Specific rock formations are determined by the type of environment existing when the rock was being formed. For example, most limestones represent marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or a riverbed.

The study and comparison of exposed rock layers (strata) in various parts of the earth led scientists in the early 19th century to propose that the rock layers could be linked together from place to place. In a local region like a country, physical characteristics of rocks can be compared and correlated. On a larger scale, even between continents, fossil evidence can help in correlating rock layers. The Law of Superposition, which states that in a horizontal sequence of rocks that has not been disturbed, the oldest rock layers will be on the bottom, with successively younger rocks on top of these, helps geologists correlate rock layers around the world. This also means that fossils found in the lowest levels in a sequence of layered rocks represent the oldest record of life there. By matching partial sequences, the truly oldest layers with fossils can be worked out.

By linking together fossils from various parts of the world, scientists are able to give relative ages to particular layers. This is called relative dating. Relative dating tells scientists if a rock layer is “older” or “younger” than another. This would also mean that fossils found in the deepest layer of rocks in an area would represent the oldest forms of life in that particular rock formation. In reading earth history, these layers would be “read” from bottom to top or oldest to most recent. If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated layers. By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale. This relative time scale divides the vast amount of earth history into various sections based on geological events (continental drift, mountain-building, and glacier movement), and notable biological events (appearance, evolution, or extinction of certain life forms).

Objectives: When you complete this activity, you will be able to: (1) sequence information using items which overlap specific sets; (2) relate sequencing to the Law of Superposition; and (3) show how fossils can be used to give relative dates to rock layers.


  • set A: nonsense syllables
  • set B: sketches of fossils
  • pencil
  • paper

Procedure for Set A:

  1. Spread the cards with the nonsense syllables on the table and determine the correct sequence of the eight cards by comparing letters that are common to individual cards and, therefore, overlap. The first card in the sequence has “Card 1, Set A” in the lower left-hand corner and represents the bottom of the sequence. If the letters “T” and “C” represent fossils in the oldest rock layer, they are the oldest fossils, or the first fossils formed in the past for this sequence of rock layers.
  2. Now, look for a card that has either a “T” or “C” written on it. Since this card has a common letter with the first card, it must go on top of the “TC” card. The fossils represented by the letters on this card are “younger” than the “T” or “C” fossils on the “TC” card which represents fossils in the oldest rock layer.
  3. Sequence the remaining cards by using the same process.
  4. When you finish, you should have a vertical stack of cards with the top card representing the youngest fossils of this rock sequence and the “TC” card at the bottom of the stack representing the oldest fossils.

Interpretation Questions:

  1. After you have arranged the cards in order, write your sequence of letters (using each letter only once) on a separate piece of paper. Starting with the top card, the letters should be in order from youngest to oldest.
  2. How do you know that “X” is older than “M”?
  3. Explain why “D” in the rock layer represented by DM is the same age as “M.”
  4. Explain why “D” in the rock layer represented by OXD is older than “D” in the rock layer represented by DM.

Procedure for Set B:

  1. Carefully examine the second set of cards which have sketches of fossils on them. Each card represents a particular rock layer with a collection of fossils that are found in that particular rock stratum. All of the fossils represented would be found in sedimentary rocks of marine origin. Figure 2-A gives some background information on the individual fossils.
  2. The oldest rock layer is marked with the letter “M” in the lower left-hand corner. The letters on the other cards have no significance to the sequencing procedure and should be ignored at this time. Find a rock layer that has at least one of the fossils you found in the oldest rock layer. This rock layer would be younger as indicated by the appearance of new fossils in the rock stratum. Keep in mind that extinction is forever. Once an organism disappears from the sequence it cannot reappear later.
  3. Use this information to sequence the cards in a vertical stack of fossils in rock strata. Arrange them from oldest to youngest with the oldest layer on the bottom and the youngest on top.

Interpretation Questions:

  1. Using the letters printed in the lower left-hand corner of each card, write the sequence of letters from the youngest layer to the oldest layer (i.e., from the top of the vertical stack to the bottom). This will enable your teacher to quickly check whether you have the correct sequence.
  2. Which fossil organisms could possibly be used as index fossils?
  3. Name three organisms represented that probably could not be used as index fossils and explain why.
  4. In what kinds of rocks might you find the fossils from this activity?
  5. State the Law of Superposition and explain how this activity illustrates this law.

Figure 2-A. Sketches of Marine Fossil Organisms (Not to Scale)

Brachiopod Trilobite Eurypterid
NAME: Brachiopod
PHYLUM: Brachiopoda
DESCRIPTION: “Lampshells”; exclusively marine organisms with soft bodies and bivalve shells; many living species
NAME: Trilobite
PHYLUM: Arthropoda
DESCRIPTION: Three-lobed body; burrowing, crawling, and swimming forms; extinct
NAME: Eurypterid
PHYLUM: Arthropoda
DESCRIPTION: Many were large (a few rare species were 5 feet in length); crawling and swimming forms; extinct

Graptolite Horn coral Crinoid
NAME: Graptolite
PHYLUM: Chordata
DESCRIPTION: Primitive form of chordate; floating form with branched stalks; extinct
NAME: Horn coral
PHYLUM: Coelenterata (Cnidaria)
DESCRIPTION: Jellyfish relative with stony (Cnidaria)(calcareous) exoskeleton found in reef environments; extinct
NAME: Crinoid
PHYLUM: Echinodermata
DESCRIPTION: Multibranched relative of starfish; lives attached to the ocean bottom; some living species (“sea lilies”)

Placoderm Foraminifera Gastropod
NAME: Placoderm
PHYLUM: Vertebrata
DESCRIPTION: Primitive armored fish; extinct
NAME: Foraminifera (microscopic type)
PHYLUM: Protozoa (Sarcodina)
DESCRIPTION: Shelled, amoeba-like organism
NAME: Gastropod
PHYLUM: Mollusca
DESCRIPTION: Snails and relatives; many living species

Pelecypod Ammonite Ichthyosaur
NAME: Pelecypod
PHYLUM: Mollusca
DESCRIPTION: Clams and oysters; many living species
NAME: Ammonite
PHYLUM: Mollusca
DESCRIPTION: Squid-like animal with coiled, chambered shell; related to modern-day Nautilus
NAME: Icthyosaur
PHYLUM: Vertebrata
DESCRIPTION: Carnivore; air-breathing aquatic animal; extinct

Shark's tooth
NAME: Shark’s tooth
PHYLUM: Vertebrata
DESCRIPTION: Cartilage fish; many living species


Figure 2-B. Stratigraphic Section for Set B
Figure 2B

Set A
Figure 2C
Set B
Figure 2D
Figure 2E