How Do We Know So Much About Life?

Geologic time can be estimated in many ways. For example, rock sequences, matching up fossils and radiometric dating.

Geologic Time

An artist's idea of what early Earth looked like.

The Earth is really old. Really, really, really old. We’ve already seen that scientists estimate it’s about 4.6 billion years old. That’s much older than you can think back. But what’s even more interesting is the fact that it was so hot that there wasn’t even any solid rock for about 500 million years. This means that Earth was a ball of boiling hot liquid with no land to speak of. After a few million years, the outer layer of the Earth cooled down enough to be solid (like the crust of a pizza) but the inner part of the Earth was still hot liquid magma. Of course, that magma wanted to get out of the middle of the Earth. So, the magma bubbled up through the crust (these are volcanoes), and so gases and water vapor ended up coming out of the hot Earth and landing in the atmosphere. That water vapor cooled down enough to become the ocean, which is where it’s believed life began.

A sample rock sequence

So there were these tiny, microscopic living things in the ocean. Amazingly, some of them actually turned into fossils so that scientists know that at least 3.5 billion years ago, these small organisms existed! Furthermore, scientists find these organisms inside of certain types of rocks that only existed during a certain time period. By doing all sorts of chemical testing, scientists can estimate how old a particular type of rock is, and once they know that, they know how old the fossils are inside of that rock. The guides which help scientists all over the world figure out how old rocks are is called a rock sequence.

One of the tests that scientists can do to figure out the age of rock is called radiometric dating. Radiometric dating uses processes that happen naturally to be able to tell how old certain things are. For example, you have a squirrel and an apple tree. Well, this squirrel happens to like eating apples, but will only eat one per day and leaves behind the core of the apple. One day, you happen to go past your apple tree and notice that there are 90 apples still on the tree and 10 apple cores on the ground. Assuming that nothing else happened with the apples or the cores, you know that the squirrel has been eating apples for 10 days. If you went by the tree seven days later, you would expect to see 17 apple cores instead of 10.

Radiometric dating is very much like this. There are certain chemicals, like uranium, which are radioactive and break down over time. They actually form other chemical elements, as uranium will form lead, just like the squirrel in our example turned the apple into a core. Scientists know how long it takes for uranium to break down into lead, so if they find some uranium and lead together, they can figure out how long that uranium has been around, and therefore how old the rock is. For example, if 90% of the rock is uranium, then it’s older than rock that’s 95% uranium.

1. What is a fossil?
2. What can radiometric dating tell scientists?
3. Is there just one rock sequence? Why or why not?
Put it together
4. If you compare the rock at the bottom of a rock sequence with the rock on top, what do you know about how old the rock at the bottom is?
5. Carbon-14, which is an unstable chemical, breaks down to form carbon-12. If sample A has a higher percentage of carbon-14 than sample B, then what do you know about the age of sample A?
Think about it
6. Create a rock sequence (like the sample from the reading) from the following information:
- Scientists found four layers of a rock sequence in Cairo, Egypt
- A fish fossil, Andrea, was found in a layer of conglomerate
- The layer of granite was the oldest rock found in Cairo
- A dinosaur fossil, Beth, was found in a layer of limestone
- A layer of shale was found in between the conglomerate and limestone
- Beth was found to have a lower percentage of carbon-14 than Andrea

8. Did humans evolve from monkeys? Explain!
9. List at least four genetic characteristics.
10. Which classification is:
a) The most general?
b) The most specific?
Rocks Tell Tale of Early Warm Atmosphere


Abundance A great deal of
Greenhouse Gas Any gas that causes heat to stay inside the atmosphere
Methane A gas that is often burned and is one of the greenhouse gases
Geologic Footprint The record of what has happened on Earth that is buried in rocks
Field Geologist Someone who studies the Earth outside of a laboratory and actually out at sites like mountains, rivers and forests
Analysis Using the results of an experiment to come to some conclusions about the experiment
Sediment Tiny rocks that fall out of water and gather on the floor or a river, lake or ocean
Regeneration The process that creates something over again

By Dawn Levy

Stanford News Service

If a time machine could take us back 4.6 billion years to the Earth’s birth, we’d see our sun shining 20 to 25 percent less brightly than today. Without an earthly greenhouse to trap the sun’s energy and warm the atmosphere, our world would be a spinning ball of ice. Life may never have evolved.

But life did evolve, so greenhouse gases must have been around to warm the Earth. Evidence from the geologic record indicates an abundance of the greenhouse gas carbon dioxide. Methane probably was present as well, but that greenhouse gas doesn’t leave enough of a geologic footprint to detect with certainty. Oxygen wasn’t around, indicate rocks from the era, which contain iron and carbon instead of iron and oxygen. Stone fingerprints of flowing streams, liquid oceans and minerals formed from evaporation confirm that 3 billion years ago, Earth was warm enough for liquid water.

Now, the geologic record revealed in some of Earth’s oldest rocks is telling a surprising tale of collapse of that greenhouse — and its subsequent regeneration. But even more surprising is the critical role that rocks played in the evolution of the early atmosphere.

Geologists Mike Tice, left, and Don Lowe display rocks that point to a warm ancient Earth with more carbon dioxide in its atmosphere than today, which likely supported a surface temperature of 70 degrees Celsius (158 F).

“This is really the first time we’ve tried to put together a picture of how the early atmosphere, early climate and early continental evolution went hand in hand,” said Donald R. Lowe, a professor of geological and environmental science who wrote the paper with Michael M. Tice, a graduate student investigating early life. “In the geologic past, climate and atmosphere were really [heavily] influenced by development of continents.”

The record in the rocks

To piece together geologic clues about what the early atmosphere was like and how it evolved, Lowe, a field geologist, has spent virtually every summer since 1977 in South Africa or Western Australia collecting rocks that are, literally, older than the hills. Some of the Earth’s oldest rocks, they are about 3.2 to 3.5 billion years old.

“The further back you go, generally, the harder it is to find an [unchanged] record, rocks that haven’t been twisted and squeezed … and otherwise altered,” Lowe says. “We’re looking back just about as far as the sedimentary record goes.”

After measuring and mapping rocks, Lowe brings samples back to Stanford to cut into sections so thin that their features can be revealed under a microscope. Scientists participate in analyses that further reveal the rocks’ histories.

The geologic record tells a story in which continents removed the greenhouse gas carbon dioxide from an early atmosphere that may have been as hot as 70 degrees Celsius (158 F). At this time the Earth was mostly ocean. It was too hot to have any ice caps. Lowe hypothesizes that rain combined with carbon dioxide to make carbonic acid, which caused the erosion of mountains of newly formed continental crust. Carbonic acid formed bicarbonate, which was carried down rivers and streams to be left behind as limestone and other minerals in ocean sediments.

Over time, great slabs of oceanic crust were pulled down, or subducted, into the Earth’s mantle. The carbon that was locked into this crust was essentially lost, tied up for the 60 million years or so that it took the minerals to get recycled back to the surface or passed through volcanoes.

The hot early atmosphere probably contained methane too, Lowe says. As carbon dioxide levels fell, the amount of carbon dioxide and methane became about equal. This blocked out light and caused further cooling, perhaps a temperature drop of 40 to 50 degrees Celsius (100 F). Eventually, about 3 billion years ago, the greenhouse just collapsed, Lowe and Tice theorize, and the Earth came into its first ice age 2.9 billion years ago.

The rise after the fall

Here the rocks reveal an odd twist in the story — regeneration of the greenhouse. Recall that 3 billion years ago, Earth was essentially Waterworld. There weren’t any plants or animals to affect the atmosphere. Even algae hadn’t evolved yet, only small unicellular organisms that produced some methane. Carbon continued to be subducted into what Lowe calls “a big storage facility … that kept most of the carbon dioxide out of the atmosphere.”

But as carbon dioxide was removed from the atmosphere and put into rock, there was less carbonic acid to erode mountains and the mountains were becoming lower. But volcanoes were still spewing into the atmosphere large amounts of carbon from deep within the ocean.

“So eventually the carbon dioxide level climbs again,” Lowe says. “It may never return to its full glorious 70 degrees Centigrade level, but it probably climbed to make the Earth warm again.”

Over the past few million years we have been going back and forth between ice and non-ice ages, Lowe says. We are in a non-ice age right now. It’s a transition — and scientists are still trying to understand how big global climate change caused by humans in recent history is compared to that caused by nature over the ages

“If we can analyze ancient climates, atmospheres, and life, we can take some first steps at understanding what is happening today and likely to happen tomorrow.”


  1. What could have caused life to never have evolved?
  2. When did Lowe and Tice think that the Earth was warm enough for liquid water?
  3. How old are the oldest rocks found on Earth?
  4. What chemical caused the mountains to erode on early Earth?
  5. What caused the greenhouse collapse that led to Earth’s first ice age 2.9 billion years ago?
  6. What made the levels of carbon dioxide go up again after the first ice age?
  7. Make a diagram showing the inside and outside of the Earth, along with a mountain, volcano and clouds. Use arrows and labels to demonstrate where carbon came from and went to inside and outside the Earth in this article.

All living things are made up of a few main chemical elements, like carbon. When living things die, many different things can happen to them. For instance, they can be broken down by decomposers, eroded by water or wind, or even crushed by other animals. Very rarely, something happens called fossilization, when the carbon inside the now dead organism hardens into what we call a fossil.

For each of the six boxes, do the following (refer to the time scale):

  1. What era and period do these fossils come from?
  2. What age range are these fossils?
  3. Pick three of the fossils, sketch them and write down the scientific name. For each fossil, find a commonname by looking on the internet, looking through the biology book, or asking someone who has already investigated this fossil.

    Geologic Time Scale

Musical Time Scale
  1. Choose your favorite musical genre.
  2. Like the geologic time scale in the book, make a time scale for your genre, including at least two eras and five periods.
  3. Make sure to include dates (years) along the side.
  4. For each of three periods, respond:
    1. What is an example of the music of this period?
    2. How would you know if you found a song you had never before heard from this period? In other words, what characteristics would these songs have?
Comparing the Domains

Until the twentieth century, classifying life forms was relatively simple. Most biologists classified living things as either plant or animal. Then, in the 1950s, the picture became more complicated as scientists looked more closely. By the 1970s, there were five kingdoms. Four were characterized by eukaryotic cells: plants, animals, fungi, and protists. The bacteria were the one group distinguished by prokaryotic cells. Looking even more closely, scientists realized that a certain group of bacteria had cells that didn’t really fit with bacteria or with the eukaryotes. The five kingdoms were replaced by six kingdoms and three domains.

This table compares the three domains and six kingdoms. Use the information in the table to answer the Analyze and Conclude questions on the next page.

Classification of Living Things





Kingdom Eubacteria Archaebacteria Protista” Fungi Plantae Animalia
Cell type Prokaryote Prokaryote Eukaryote Eukaryote Eukaryote Eukaryote
Cell structures Cell walls with peptidoglycan Cell walls without peptidoglycan Some cellulose cell walls; chloroplasts in some Cell walls of chitin Cell walls of cellulose, chloroplasts No cell walls, no chloroplasts
Number of cells Unicellular Unicellular Most unicellular; some multicellular; some colonial Most multicellular; some unicellular Most multicellular; some green algae unicellular Multicellular
Mode of nutrition Autotroph or heterotroph Autotroph or heterotroph Autotroph or heterotroph Heterotroph Autotroph Heterotroph
Examples Streptococcus, Escherichia coli Methanogens, halophiles Paramecium, amoeba, giant kelp, slime molds Mushrooms, yeasts Mosses, ferns, flowering plants Sponges, worms, insects, fishes, mammals

Analyze and Conclude

  1. Which domain includes four kingdoms? What are those kingdoms?
  2. Which kingdom has cells that lack cell walls?
  3. Which domain includes multicellular organisms?
  4. How do all members of domain Eukarya differ from all members of domains Archaea and Bacteria?
  5. On the basis of the information in the table, how are the members of domain Archaea similar to those of domain Bacteria?

Build Science Skills

If you were to observe a multicellular organism without cell walls while looking under a microscope, in which domain and kingdom would you classify it? Explain your reasoning.