Phylogeny Tracker: Clades from Biota to Animalia

In this series we trace interesting phylogenetic relationships between clades of organisms. This is mostly a fun exercise in taxonomy, but it also helps demystify the complex relationships between various groups of organisms.

Today we are looking at the path that leads from Biota, the root of life, all the way to animals.

Follow along by using the Phylogeny Explorer. Click along the indicated links in the tree so you can eventually get to Animalia.

We first have to take the Eukaryota clade. All animals consist of multiple cells with a nucleus, so they are eukaryotes.

Animals cells have either one or no flagellum, so they are unikonts. Animals also have a triple-gene fusion only in Unikonta. So therefore, we must now turn down the Unikonta clade.

Next we turn down the Obazoa clade. Not much is known about this understudied clade, but it includes animal and fungi, so down this clade we must go.

The flagellate cells of animals, such as sperm, propel themselves with a single posterior flagellum. This is a common characteristic of the Opisthokont clade. Fungi are in this clade, but some have lost their flagellum.

Next we must go down the Holozoa clade. This includes all animals and their most closely related unicellular sisters (e.g. choanoflagellates). But not fungi. Most unicellular holozoans are parasites.

Animals cells maintain the slender thread-like projections or “tentacles” of the ancestral Opsithokont cell, so now we must head down the Filozoa clade.

Both animals and choanoflagellates are able to form multicellular units. Animals are permanently multicellular, while choanoflagellates can form multicellular colonies. So now we head down the Apoikozoa (“Colony Animals) clade.

Sponges are animals too, albeit primitive ones that are sessile for most of their life cycle.

Finally we reach Metazoa, or Animalia. What are the characteristics of animals? Many. We will cover over some of the more readily grasped ones.

They are eukaryotic and multicellular. Unlike plants and algae, which produce their own nutrients, animals are heterotrophic, that is they cannot produce their own food and must ingest it from an external source.

With few exceptions, they respire aerobically. That is they require oxygen in order for cells to produce oxygen.

All animals are motile at some stage of their life-cycle, including the sponge. That is, at some stage of their life cycle, they are mobile. Although, later in life they may become sessile (unable to move by their own power).

Alright, that will do for our brief coverage of the key characteristics of animals. We have seen that to get to Animalia, we have to pass through several basic clades.

Next time we will start off in the Animalia clade and see how long it takes to get to Thelodonti, that is a clade of jawless “fish” with distinct scales, rather than large armor plates. We will have to pass through more than ten clades to reach this point.

It will take us a few just to get to Chordata (animals with a notochord), which include the vertebrates!

Stay tuned.

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