1. Dear ladies and gentlemen, Dear students, It gives me great pleasure to take part in this meeting, so I would like to thank the organizing committee for inviting me to this lecture. I am going to talk about the evolution of the nervous system. But firstly I need to point out that this title does not imply that the evolution of the body and the evolution of the organs are separate things.
2. I think that one of the most important scientific discoveries this year is presented in Yang J et al. ’s article, which is published in PNAS. I say this because so far, findings on the evolution of the nervous system have been based on cranial bone tissue. It seems impossible that remains of the brain or nervous system could be found in fossils, but Yang J et al. reveal that the oldest nervous system remains are found in Panarthropod fossils, which are nearly 500 billion years old.
3. In this article we can clearly see the nervous system plan of a 500 billion year old creature. The chain of ganglions and the cranio-caudal organization and a differentiation in the cranial part of the creature. So, the story of the nervous system should cover at least this period. Consequently, this story should be more than 500 billion years old.
4. I am a neurophysiologist, so I am interested in the excitablity of cells. In my classes, I say that a neurologist is a doctor of excitable cells. In our bodies we have two kinds of cell, excitable and non excitable. The main difference between these two kinds of cell is the ability to change polarity. The main feature of excitable cells is the ability to rapidly change their membrane potential. The membranes of the cells have potential differences, therefore they have polarity and are able to change this polarity very rapidly; they can depolarize and repolarize.These very unique features are due to their membrane channels, voltage dependent Na channels. Excitable cells express voltage dependent Na channels.
5. Voltage dependent channels have 4 units and every unit has 6 transmembrane domains. They allow Na to flow according to the membrane potential.
6. So the story of the evolution of the nervous system begins with the evolution of voltage dependent Na channels. Voltage dependent Na channels are believed to have evolved from calcium channels at the origin of the nervous system. Voltage dependent K channels were the first voltage dependent membrane channels and either Ca or Na channels evolved from 6 trans membrane K channels by duplication of their genome.
7. Recent experiments suggest that Voltage gated Na channels are related to the emergence of Metazoa.
8. This is our tree of life and we know that only metazoa have voltage gated Na channels, which evolved from the voltage gated K channels in bacteria.
9. And probably one of the most important connections in evolution is the emergence of the nervous system draft, which can only be seen in multicellular animals.
10. I am sure you appreciate the difficulty of explaining the evolution of the nervous system using this huge tree, which contains all the branches. I recommend this nice link.
11. The subject is over my head.
12. Above all, dealing with the human brain will require much more time and much more effort. So I am going to give a bird’s-eye view.
13. I like this kind of view because it contains roots and it does not categorize by hierarchy.
14. This cladogram is my main road in this presentation. The first junction is at the divergence to the vertebrates.
15. The draft of the nervous system has several plans. The simplest form of network plan is seen in Hydras.
16. This plan is really a simple network which contains only one kind of nerve cell.
17. But this network manages to execute very complicated movements.
18. The Amphioxus also has a very simple nervous system draft. It is not a vertebrate but vertebrates have the same ancestors as the Amphioxus.
19. In Turkish we call the Amphioxus ‘Batrak’. It has a craniocaudal longitudinal nervous system plan.
20. This nervous system belongs to the simplest living chordate and its main feature is the sharp point at each end.
21. In vertebrates, the chordate has a new cranial organization.
22. The simplest vertebrates are chordates and also have a complex cranial organization. The new neuron cumulation and communications are developed. They are jawless fish.
23. Lampreys are jawless fish.
24. And in Turkish we call these animals «Taşemen». Their nervous system has a diencephalon, mesensephalon, telencephalon and olfactory bulb.
25. Cartilaginous fish have more complex nervous systems.
26. There is a new neuron organization relating to equilibrium during movement. It is the cerebellum.
27. Sharks’ nervous systems have the same cerebellum organization.
28. Dogfish are also hunters and their telencephalon is profoundly developed.
29. Bony vertebrates are divided into two main branches; Ray-finned vertebrates and Lobe-finned vertebrates. I will quickly skip through ray-finned fish and move on to teleosts.
30. Electric fish have an enormous cerebellum. This is probably to detect the electrical fields of objects.
31. Mormyrids are an example of mosaic evolution in teleost fish.
32. Salmon;
33. Salmon have a basic nervous system plan, like other fish.
34. Polypterus;
35. Polypterus has a rather primitive plan but the draft of the nervous system is similar to that of other fish.
36. Polypterus has a rather primitive plan but the draft of the nervous system is similar to that of other fish.
37. As their branch’s name suggests, their fins seem to be an extension of the body, but they have very a small brain.
38. The latest genomic studies also support the theory that Coelacanths are living fossils and that Lungfish (not Coelacanths) are the closest relatives to Tetrapods.
39. Even at this stage, where we are approaching the Tetrapods, we are still 350 million years from today.
40. Lungfishes; We have reached another very special road junction. Their air sacs are going to turn into lungs.
41. Lungfish, especially those from Africa, look like eels with tiny toeless legs. Their brains as well as their bodies, exhibit many features that are typical of very young animals in other species.
42. Another junction is related to tetrapods.
43. The Salamander is a tetrapod animal. Its extremities also have toes, but its brain model is similar to that of a lungfish.
44. I am going to follow the path of Amniotes because this junction is one of the main stages in nervous system evolution.
45. Opossums are marsupials with relatively small and simple brains. But the main nervous system plan is very similar to that of vertebrates.
46. Turtles;
47. Turtles belong to another branch and their brains are interesting because they can function with very little oxygen.
48. Another main branch is Birds;
49. Ravens are the largest living songbirds, they have remarkably large brains and are surprisingly intelligent. As you can see, the telencephalon is very big if you compare it to other brain parts.
50. Placentals; at last we come to the end of this cladogram.
51. As a summary, the Vertebrate nervous system has a basic organization plan; Medulla spinalis, spinal nerves, 12 cranial nerves, medulla oblongata, pons, mesencephalon, cerebellum, telencephalon etc.
52. If you compare it with the human brainstem you can see the same draft; Medulla spinalis, spinal nerves, 12 cranial nerves, medulla oblongata, pons, mesencephalon, cerebellum, telencephalon etc.
53. And the main difference is proportionally telencephalic enlargements.
54. On this marvellous road we have passed some important junctions; vertebrates, bony vertebrates, lobe-finned vertebrates, tetrapods, amniotes, mammals, placentals…..
55. And we still have a very long road ahead to understand nervous system evolution. We can see that common cortical fields have been identified, from placentals to the human in this phylogenetic tree illustration. Every species has primary association areas but secondary and tertiary association areas vary proportionally among species.
56. Lastly I would like to share this tree; branches of Old World monkeys; Variations in the telencephalon and cortical areas determine the reorganisation of the central nervous system.
57. As I said at the beginning, we should keep in mind that the body and brain evolve together. This famous figure shows this relationship in a logarithmic scale. All animals obey this rule. As you see, even if there are some extreme deviations such as humans.
58. Thank you very much for your attention.
Prof.Dr. Hilmi Uysal, October 2016.