By Robert Hazen, George Mason University
By far, the most compelling evidence for evolution comes from the broad-brush changes of life that we see during the last 4 billion years, looking at the different layers of rock. Earth’s history is divided into several long time periods, based on the distinctive characteristic of rocks and fossils from those periods. We can review life’s history, eon by eon, as revealed in the rocks.
What Are Fossil Records?
The primary source of evidence for evolution is fossils; it’s the fossil record, the unambiguous testimony of the rocks. Fossils are evidence of past life. They’re usually preserved in layers of sedimentary rock. Most fossils are preserved as hard parts—the hard parts of animals, or of plants, such as shells, bones, wood, that sort of thing.
The fossil record refers to all the varied fossils that have been unearthed all over the world. The record is vast and constantly growing. It’s remarkable in the clarity of the portrait it gives of how life changed over time.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Precambrian Period: When Life Began to Evolve
The earliest period of Earth history is the Precambrian Period. It includes all the rock formations older than 570 million years; that’s before the appearance of animals that had hard parts. Thus, the Precambrian period spans about 85 percent of Earth’s history; the first 4 billion years, roughly. We divide the Precambrian into other eras.
From about 4.5 to 4 billion years ago, the earliest period, which is sometimes called the Hadean Era, that was when the great bombardment slowly subsided. We think that the last great globally sterilizing event, the last great impact that would have wiped out the atmosphere and the oceans and killed every living thing, at least on the surface, was probably about 4 billion years ago.
That’s when life may have begun to evolve in a significant way. Subsequently, there was intense volcanism; that replenished the oceans, it would have replenished the atmosphere with gases. This, then, was the time that we think chemical evolution probably occurred.
Appearance of Single-celled Organisms
We don’t really know when life began, but we are quite sure that the oldest rocks on Earth, which date from 3.8 to 3.5 billion years old, contain clear signs of single-celled organisms. If we make thin slices of these rocks, we can look at them in a microscope, and we can actually catch the microbes in the process of dividing, in some cases.
We have clear evidence that cell division was going on, and that microbes were around at that point, so we know that life must have arisen very soon after the end of the great bombardment.
Signs of Multicellular Life
We then have rocks from about 2.5 billion to 1.0 billion years ago, and these reveal an increasing diversity of single-celled life forms, and the first signs of multicellular life. We see strands of cells that are identical to modern blue-green algae, for example; and they suggest the beginnings of an oxygen-rich atmosphere, the beginning of the production of atmosphere, by about 2 billion years ago.
Then, about 1 billion years ago, there were a number of dramatic changes that we begin to see in the fossil record. We start seeing possible worm burrows at about this time. Also, the first fossil jellyfish, and other multicellular organisms, seem to appear at about 1 billion years in age.
Multicellular Ocean Life
The most dramatic change in the rock record occurs, for this early period, at about 700 million years ago, which suggests that there was a period in Earth history where there may have been a period of very extreme cooling; in fact, so extreme that all the oceans may have frozen over. No one is sure exactly what happened at that period 700 million years ago, but it does represent a dramatic change in Earth climate, and that’s the subject of a lot more study.
Following what may have been then a great thawing of the oceans, there was an abundance of soft-bodied, multicellular ocean life that appeared; these are found preserved as impressions in various rock layers around the world. While there may have been multicellular life, nevertheless, if you were transported back to that early ocean, say 600 million years ago, it would have seemed very, very alien.
The surface of the Earth would be more or less barren, maybe just green scum coating some of the rock surfaces. The oceans would have none of the abundance and variety of life that we see today—very few recognizable sea creatures, at that time.
Common Questions about Early Life as Seen in Fossil Records
The earliest period of Earth history is the Precambrian Period. It includes all the rock formations older than 570 million years; that’s before the appearance of animals that had hard parts. Thus, the Precambrian period spans about 85 percent of Earth’s history; the first 4 billion years, roughly.
We don’t really know when life began, but we are quite sure that the oldest rocks on Earth, which date from 3.8 to 3.5 billion years old, contain clear signs of single-celled organisms. We have clear evidence that cell division was going on, and that microbes were around at that point, so we know that life must have arisen very soon after the end of the great bombardment.
About 700 million years ago, there may have been a period of very extreme cooling which might have left all the oceans frozen. Following what may have been then a great thawing of the oceans, there was an abundance of soft-bodied, multicellular ocean life that appeared; these are found preserved as impressions in various rock layers around the world.
