Back and outward from humans
Organisms that we are familiar with—cats, rats and elephants—are related to one another on the tree of life, often pictured as a phylogeny. They are not on a ladder of life from simplest to most complex, not in a linear order. If we were to survey the diversity of organisms alive today on Earth, we couldn’t start at one end and march through from one to the next and then arrive at humans. Humans are not at the top, when it comes to phylogenetic relationships. No organism is at the top. And if we restrict ourselves to organisms living today, none is at the base either. We are all just cousins, more or less related to one another. When you think about it for a minute, it doesn’t make sense to call any organism ‘unrelated’ to us. Oak trees are related to us, they are just a lot less closely related to us than acorn woodpeckers.

Figure 1. Scala naturae as depicted by Ramon Llull (1305) in “Ladder of Ascent and Descent of the Mind”.

Aristotle, medieval philosophers, and naturalists a century before Darwin often tried to discern a linear scala naturae. Piecing together bits from Aristotle, it would go something like this: rocks, fungi, plants, worms meaning ‘invertebrates’, fish, whales and seals, reptiles, beasts on four legs, Man. But there’s something wrong with this linearization. Can you think of a reptile that has some fantastic special adaptations of its own? How about a whale? How about an invertebrate? And then there are plenty of plants and fungi that have amazing derived features. Just consider how whales sign songs to one another over long distances.

Books and courses in Canvas are typically arranged in a line. You learn things along the arrow of time. If you write something down for someone else, they will read it linearly. Therefore, we want to develop your knowledge of organismal diversity not as it actually is, rather from a much more biased perspective. We choose to teach you about life’s diversity from the perspective of humans. To do this, we make this perspective a retrospective. Stand as a human and look backwards in time. Look backward to the co-ancestor of humans with our closest cousins. Our closest living cousins are the chimpanzees, of which there are two species, the Common Chimp (Pan troglodytes) and the Bonobo (Pan paniscus). We are equally closely related to the Common Chimp and to the Bonobo. They are more closely related to each other than they are to us. We differ from them in a few ways—we speak languages that are infinitely recombinable, whereas their communications are much more limited; we stand upright in a precarious posture, whereas they are much sturdier knuckle walkers; we are sparsely hairy, whereas they are somewhat hairier. Nevertheless, we share a great deal in common with chimpanzees. This is because we and they all share a common ancestor who lived roughly 6 million years ago. From that common ancestor humans and chimps inherited a number of shared traits.

Now, look a bit further back in time to the confluence of our lineage with that of gorillas. Our (and chimps) most recent common ancestor with gorillas lived on the order of 8 million years ago (approximately). There are two species of gorilla, so the common ancestor of the African Apes gave rise to five species that are currently alive (two types of gorillas, two types of chimps, and all humans). The African Apes is the name of the clade that contains these five living species. It also contains some other species that are now extinct and that are poorly known from a few fossils. In undertaking our retrospective, let us ignore extinct forms, for the sake of simplicity. Only consider organisms that are alive today.

You can continue this tale of ancestors moving backwards in time from humans to our deep ancestors, and simultaneously moving outwards from humans to our more and more remote cousins. A good tool for looking up this information is

http://www.onezoom.org/ (Links to an external site.)

Start by searching for “humans” and then zoom out. Onezoom ignores extinct forms, which greatly simplifies the lesson that we are trying to have you learn. Go backwards from humans and at each confluence, write down (a) the clade that our lineages is merging with or some example in that clade, (b) the date of the confluence, i.e., the estimate of how many millions of years ago the common ancestor lived, (c) the name of the larger clade encompassing both branches it gave rise to if it is named, (d) the number of species in the larger clade. Write this on a long piece of paper or a Word document, starting at the top like so:

1-chimps 6 MYA unnamed 3 species
2-gorillas 8 MYA African Apes 5 species
3-etc.

Continue the ancestors’ tale back to and including the common ancestor with Fungi. How many confluences of still-living clades were there from humans to that point? In other words, how many lines of data are on your paper? Are you surprised at how large the number is, or how small, or is it just what you would have guessed? How many confluences of still-living clades were there from humans back to the common ancestor of all Vertebrates? How about to the common ancestor of all Primates?

Back and outward from some other organism
Now, do the same thing with some plant or animal other than a vertebrate. It could be an oak tree or a fruit fly or whatever you can find in onezoom. As you go take notes about the clades that are joining with your focal clade as you go. What are the distinctive features of co-ancestors? Please note: the tree-of-life that biologists have inferred is not completely resolved—not even close!—and onezoom is presenting a completely resolved, always bifurcating phylogeny. To do this, onezoom is guessing at one of various possible ways in which the phylogeny might have bifurcated. Be sure not to believe each phylogenetic detail, especially for organisms that are poorly studied.

What do you make of it?
After you have done this exercise and thought about it, you can write a paper. Some of your paper can concisely narrate the two ancestors’ tales. However, you should also explain what it means to you. In what sense do we object to the distinction between ‘higher (advanced) animals’ and ‘lower (primitive) animals’? How does the concept of the “ladder-of-life” or scala naturae fail? How about cartoons like the following?