Part 10: Layers in a trifle
After taking a break from my Evolution and Me series, I've decided to resume with this post by considering a trifle. The interesting thing about this desert is all those layers – they all tell a story. My mom, for example, might make a trifle as follows: first at the bottom of the bowl, add a layer of finger biscuits or sponge cake. Next add a layer of custard, then fruit, then jam, then mince. Finally, top it all off with some whipped cream (yum!)
Next time you sit down, ready to enjoy this desert, look at the various layers and ponder the question: which layer was placed down first? It only seems logical that the bottom most layer in the above example (i.e., the finger biscuits) was first, followed by every consecutive layer. The uppermost layer, the whipped cream in this case, is logically the youngest layer of the lot. If you understand this, then you understand what geologists call the Principle of Superposition: that any layer must be older than the one above it.
Long before Charles Darwin published On The Origin of Species, geologists noticed that the Earth's rock is structured in much the same way, into distinct layers, or strata. And interestingly enough, most strata contain their own distinct fossils of living organisms now long extinct. But what is even more interesting is that these fossils, when taken as a whole, tell a very perplexing story.
Increase in complexity
One element of this story is that the very earliest layers contain the remains of simple creatures, and as you work your way up to the upper layers, fossil remains generally become more complex in structure. If you accept the Principle of Superposition, you would logically conclude that simple creatures were placed down first, followed by more complex creatures. One would have to ask why complex creatures are absent in the oldest layers.
But most interestingly, fossils do not appear randomly across various layers. They seem, in many instances, to follow a pattern that suggests that fossils between layers are linked in some way. We often observe that the characteristics of newer fossils seem to be modified forms of earlier fossils. How do we explain this?
If we look at these, and many other, patterns in the fossil record, one is left pondering why we observe what we observe. I believe that evolution is the best idea that we have come up with that can make adequate sense of these patterns.
In my next post, I will focus on some of these observed linkages, examples of what palaeontologists refer to as transitional fossils.
Next post: A long line of photographs
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