As I've said before, some of the best evidence for evolution, in this case I'm mainly referring to common descent and macro evolution, comes from the nested hierarchy of the similarities of living organisms, the tree of life. To drive the point home, let's take a look at the evolution and relationships of a very large and familiar group of animals, the courtates. During this video, I'll start with the most primitive adaptations, providing fossil and living examples, and move towards more advanced adaptations. This is not to say that evolutionary relationships are like a ladder. When evolutionary biologists use the term primitive in advance to refer to adaptations, what we mean is that primitive adaptations arose earlier in time than advanced ones. In other words, the advanced adaptations are more derived in form from the ancestral state than the primitive ones are. Advanced, therefore, is an adjudgment about how good an adaptation is. So, all of the living descendants of primitive organisms are in a sense equally evolved, regardless of whether or not they have certain more advanced or derived adaptations. Again, evolution in higher organisms generally creates a pattern like a continuously growing tree, not at all like a ladder. The origin of courtates and most of the major animal filers seems to lie in the evolution of segmentation. This seems to be one of the major triggers for the Cambrian explosion and is a common theme throughout evolution. Segmentation in animals is caused by repeated copies of hox genes. The order of these genes on a chromosome determines the order of parts on a body from head to tail. Identical copies of a hox gene will produce identical segments, like we see in the segmented worms. But as these genes diverge, segments are able to differentiate, forming new parts. We have evidence of the existence of aquatic worms from a few tens of millions of years prior to the Cambrian from the fossilized remains of their holes. At some point in the early Cambrian, we begin to find worm-like, himmy courtates that have some courtate features but are still considered invertebrates. Invertebrate descendants of this group still exist today in organisms like the acorn worms. We also begin to see organisms like the Euro-cordates or tunicates and cephalocordates or Lancelots around this time period. Adult tunicates are weird looking sedentary filter feeders, but in their larval form they look like a cross between a worm and a fish. Lancelots and the larval form of lampries and hagfish look remarkably like the tunicate larvae as well. This is the most basic body plan that is still considered a courtate. The five main diagnostic characteristics of this little worm fish that are shared by all courtates at some point in their life cycle are... 1. The Notocord, from which the group gets its name, which is a tiny cartilaginous rod. Cartilage itself is simply a modified form of connective tissue not unlike that found in worms and other non-cordate animals. 2. Adorsal tubular nerve cord. During development this starts out as a flat sheet that is part of the epidermis and early on it curls into a tube. Nervous tissue is present in other animals including worms. 3. A Post-Anal Tail. In worms the anus is at the end of the tail, but in courtates some portion of tail jut out above and past the anus. 4. Forringial Gil Slits. In aquatic courtates these develop into full-fledged gills, while in others like humans they appear in embryos but are lost before birth and never become gills. Gills in general are also found in other animals including some worms. 5. An Endo Style. This is a siliated groove in the fairings that secrete mucous and catches food in cephalocortates and urocortates. In Lampri's the larvae have an endo style but it develops into the thyroid gland in adults which is the form it takes in most courtates. The gill arches common to the courtates are typically supported by cartilage or bone. It has been hypothesized that jaws evolved from the first set of gill arches. This is supported by both genetic and developmental data indicating a common origin of jaws and gill arches. Teeth, unlike bones, form from the epidermal layer during development. This isn't surprising since it appears that teeth evolved from scales. All one has to do to see that this must be the case is examine a shark. As you can see the scales of a shark are nearly identical to the teeth. The teeth are simply larger scales that form on the skin around the opening of the shark's mouth. The genes responsible for the development of gill rays and sharks, part of the structure of the cartilage and its gill arches supporting shark gills, have recently been discovered to be related to the same genes that regulate the development of fins and fish and ultimately limbs in tetrapods. The developmental pathways and structures are very similar using a lot of the same chemical signals. In fish we see a variety of different bones associated with the fins. In the lobe finned fish these form a sort of paddle that is used by some to drag themselves across land. In the fossil tiktolic we can see the origins of the tetrapod arrangement of limb bones, one long bone like the hemorrhacer femur, then two long bones side by side, the radius and ulna, or tibian fibula, then a set of smaller bones forming the wrist or ankle, and finally the philanges, fingers and toes. This pattern is seen in all of the tetrapods except for in a few groups where some of these bones are secondarily lost. In some primitive worms, blood flows passively without the aid of a heart, but in the segmented worms there are muscles lining the blood vessels that squeeze the blood through. The heart in urochordates, aka tunicates, and cephalocordates, has one chamber. Tunicates have been experimentally manipulated to express the gene that signals heart development in the tail muscles just after the heart, causing the development of a second functional heart chamber in place of a section of tail muscle. A change like this one duplicating heart chambers could occur naturally through hawks gene duplication. In fish, there are two chambers, amphibians have three, reptiles have three with a partial separation of the ventricle, this is an intermediate step between three and four chambers, and mammals, birds and crocodiles have four chambers. The blood supply to the heart is basically the same in the fish and tetrapods, which is part of why humans have so many heart problems in old age.