While everybody understands that black bears are related to grizzly bears and we can even figure they are related to extinct bears, lots of people wonder how scientists can be so sure that bears are related to salmon as well.
One evidence is rock layersspecifically, what is called the geologic column.
Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
This matching process is called correlation, which has been an important process in constructing geological timescales.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
We're not talking about an abstract diagram: this is the actual record of the earth's crust, recorded in rocks around the world.
But how do we know this evolutionary sequence of layers, one on top of the other, is accurate? Two laws, or principles of geology explain why rock layers are formed in this way.
Basically, scientists have learned that rocks are stacked in layers containing fossils with the oldest fossils at the deepest layers, and the youngest, or most recent fossils, near the top. At the bottom of the timeline there are no fossils of modern animals.
As you move towards the surface, you find fish, then amphibians, then reptiles, mammals, birds, and finally modern mammals including humans.
Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its dating.
These skeptics do not provide scientific evidence for their views.
Rock layers are usually ordered with the oldest layers on the bottom, and the most recent layers on top.
The Law of Faunal Succession explains that fossils found in rock layers are also ordered in this way.
Unlike people, you can’t really guess the age of a rock from looking at it.
Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.