One of the great things about evolution are the smooth transitions that occur between different species. Since most of the transition-species are extinct, they get very little attention (kind of part of being dead, I suppose). Here I present some of these unsung heroes of evolution. Listen carefully, because we could de-evolve into any of these slick dudes at ANY TIME:

1. Ape and Human – Sahelanthropus tchadensis

When people talk about proto-humans, species in the genus Australopithecus (e.g. “Lucy”) are the usual suspects. I personally prefer our friend Sahelanthropus tchadensis, pictured above. While S. tchadensis is much more on the ape side of evolution than it is on the human side, it has been suggested that it was fully bipedal (walks on two legs), making it a nice transition form from tree-swinging apes to hop-scotching humans.

2. Reptile and Mammal – Therapsids

I remember watching a movie in elementary school and being perplexed by the statement that mammals evolved from reptiles. It wasn’t until my comparative vertebrate anatomy class in my sophomore year that the mystery was cleared up! Therapsids, like Lycaenops (“Wolf-face”, ha!) pictured above, are quite basically mammal-like reptiles. The main features of these wolfy-lizards are: large developed jaws, a sleeker and lighter skull, and placement of the legs below the body to facilitate fast “galloping” as seen in the modern pooch. Some scientists even believe that therapsids had by this time evolved mammal-like hair (fur).

3. Fish and Amphibian – Tiktaalik

You remember the Tiktaalik, don’t you? He remembers you! Just kidding, but this squirmy little guy wins the award for Most Likely to Frustrate Believers In Intelligent Design due to his existence being predicted long before his bones were even found in 2004. Also, the Tiktaalik is Canadian (found on Ellesmere Island, Nunavut)! For that, I give him a four fleshy-lobed fin salute.

4. Invertebrate and Vertebrate – Larval Urochordate

Another difficult evolutionary step to visualize is the transition between worm-like sea beasties and fish with basic spines composed of vertebrae (i.e. vertebrates) . The leading hypothesis is actually quite ingenious and satisfying, much like this blog. It goes something like this: In the fossil record, oceanic vertebrates appear suddenly 550 million years ago. Up until to this point, the sea was dominated by crazy giant insects and Urochordates, who were (and are!) invertebrate sessile filter feeders (like a sea sponge). The key fact is that the earliest life stage (larval stage, pictured above) of the Urochordate was similar to a tadpole, and even had a rudimentary spine. The hypothesis states that some natural selection situation caused the Urochordate to stay a larva for longer and longer until eventuall, the sessile adult stage was totally eliminated (this process is called neoteny in evo-devo jargon). So in the end we were left with a nice little tadpole who eventually gave rise to all the fishies in the deep blue sea. As for mollusks and the like…I have no idea!

5. Unicellular and Multicellular Life – Cellular Slime Mold

When life first arose, it was almost definitely in the form of one-celled creatures. While many of these simple organisms have persisted in one form or another to present day, others somehow made the jump from being composed of a single cell to being composed of many. This is not a trivial evolutionary step! Now, while the theoretical background of the evolution of multicellularity is a little mind numbing and by nature non-empirical, there are existing organisms that can give us clues about how it might have occurred. For example: the weird ooze pictured about is a cellular slime mold that begins its life as a group of unicellular organisms, but eventually matures into an enormous multicellular (and gooey) individual! Weird!

6. Primordial Ooze to Unicellular Life – ???

What am I, a biochemist?