What goes on a timeline of life? November 05 2020, 3 Comments

Recently, a teacher asked me for a recommendation for a timeline of life. It had been a couple of years since I had looked for this material, and I did an Internet search to see what is available. I know of two companies that have acceptable versions of this important material, and I wanted to see if there were more.

What I found was that many materials based on the old, traditional Montessori timeline of life are still being sold. There are many mistakes on that chart, and it has serious omissions as well. I will list some of the problems, and I hope this helps teachers pick a useful timeline of life, one that has valid information and that does not implant misconceptions that children will have to unlearn.

Let’s pretend that you are looking at a timeline of life, and you wish evaluate it for your classroom. I will make my recommendations based on the use of evidence-based information in lessons for children, which I feel is extremely important.

First, stand back and take a look at the overall design. The traditional timeline had red lines running across it. Does the timeline you are evaluating have any lines that come together – converge – from older times to younger times? That’s looking from left to right on the timeline. If there are any converging lines, the timeline flunks. It will give children a misconception, not a valid impression. Life branches out through time, and the branches do not fuse back together.

If the timeline has lines that do not converge, are lines used to symbolize more than one thing? It is best to have the lines tell only one story. On the traditional timeline, one line symbolized the rise and fall of trilobite populations, and the others symbolized something else. What it is, I cannot tell. Perhaps the intent was to give a general impression of the flow of life. I don’t think that lots of flowing lines accomplish that goal very well, and they do nothing to help children understand evolutionary tree diagrams (phylogenies) that are used in biological classification.  

By the way, the trilobite line should extend to the end of the Paleozoic Era. When the first Montessori timelines were made, the later trilobite fossils had not been found. That’s why the trilobite line usually stops at the end of the Ordovician Period. Scientists have learned a great deal about the history of life in the 50 or more years since the traditional timeline was created. We need to share reasonably current information with children.

Lastly, there were a huge variety of trilobite shapes and sizes. It gives the wrong impression to show only one. Below, photos of fossils show a small sample of trilobite diversity.

Now, check the timeline for the five great extinctions. The traditional timeline marked all extinctions as ice ages, whether or not ice was a factor. There are extinctions in every geologic time period, but paleontologists recognize the five great extinctions as the largest ones and the greatest shapers of life. Greater than 50% of species became extinct as the result of those events. Those extinctions happened at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous Periods.

None of the great extinctions were caused by glaciation and cooling, although these conditions are factors in less severe extinctions. The timeline of life should show the great extinctions, but they should not be indicated by icicles. You will likely lose credibility with children if you show glaciation causing the extinction of the dinosaurs at the end of the Cretaceous Period. Many of them know about the asteroid impact at that time. If your timeline doesn’t show the Great Dying at the end of the Permian Period, it omits a very important event in the history of life.

Next, take a look at the illustrations of the organisms on the timeline. Are any of them photos or drawings of extant (currently living) organisms? If so, that timeline flunks. For example, the first trees at the end of the Devonian Period should not look like today’s trees. The first marsupial should not be a kangaroo. That is just like showing the first placental mammal as a horse. Both horses and kangaroos evolved in the late Cenozoic, not in the Mesozoic when their ancestors first lived. The goal of the timeline of life is to show children how life has changed throughout time. If they see images of modern species in prehistoric times, they may infer that life hasn’t changed much.

When the traditional timeline was created, scientists thought that trilobites were the main animal of the Cambrian Period. The older timelines say “Age of Trilobites” above the Cambrian. Since that time, paleontologists have discovered many more Cambrian fossils, and trilobites, while important, are not the most abundant. In the Cambrian, a timeline should show the unique animals of that time, such as Wiwaxia, Hallucigenia, Anomalocaris, and Pikaia, which is one of the first chordates. Children will likely be fascinated by Opabinia, a creature with five eyes and a nozzle. See https://ocean.si.edu/through-time/ancient-seas/collection-cambrian-fossils. You can use this opportunity to introduce an important principle – Life tries everything, and what works survives.

Does the timeline you are evaluating have a sand dollar in the Cambrian Period? That error appeared on the first timelines and has been copied many times. There were echinoderms living then, but the sand dollar didn’t evolve until the Cenozoic Era. Cambrian echinoderms do not resemble those of today.   

Other things I have seen in the Cambrian that do not belong there include a nudibranch. These shell-less mollusks have left little or no fossil record. It would be better to show a mollusk whose fossils date to the Cambrian. Every illustration on the timeline of life should reflect fossil evidence. The story we tell should be nonfiction.

What about showing life before the Cambrian period? It would certainly be appropriate to show the Ediacaran biota. Those organisms should at least be mentioned in the Second Great Lesson, along with the microorganisms that were really the first life.

My publication, Outline of Geologic Time and the History of Life, gives you the current ages for the geologic time periods and helps you tell the story of what happened to life through time. You can see it at https://big-picture-science.myshopify.com/collections/big-picture-science-digital-downloads/products/outline-of-geologic-time-and-the-history-of-life.

There is a lot more to the timeline, and in my next blog, I’ll address other problems in the rest of the chart.