Evidence of the origin of vertebrates Flashcards
why is our knowledge limited?
• Our knowledge of the origin and early evolution is limited:
○ Lack of intermediate fossils
○ A poor fossil record of early vertebrate forms (Cambrian through silurian period)
describe the first vertebrates
• Small approx. 3 cm
• Active predators
• Gill pouches - used gills for respiration - heavily dependent on an aquatic environment
• Had a notochord, cranium, and W shaped myomeres
• No bone
• Recent findings from early cambrian (520 mya) formations in china:
• Myllokuingia fengjiaoa
-Haikouivhthys ercaiunensis
describe Haikouivhthys ercaiunensis
- Has a cranium to protect its head
- W shape myomeres
- They both have the 5 features of chordates ?
describe the relatives of early chordates and why they were not chordates
• Not vertebrates - stripped myomeres - relative of early chordates • Pikaia - 500 mya, ○ Not a true cephalochordate (straight vs V myomeres) • Xidazoon- 520 mya ○ A "Stem chordate" ○ Expanded ○ Pharyngeal region with gill slits ○ Segmented body region ○ No notochord • Haikouella - 520 mya ○ Sister group to vertebrates ○ Large brain, eyes, branchial bars, pharynx, endostyle Lack cranium
what are the possible vertebrate ancestors?
Hemichordates, urochordates, cephalochordates
describe hemichordates (two groups)
• ○ Small, worm like marine animals ○ 3 body regions: § Proboscis, collar, trunk § Ex. Enteropneusts (acorn worms) □ Burrowing, intertidal § Ex. Pterobranchs □ Small, sessile, tube like, deep oceans
§ Both groups Have a primitive notochord (stomochord) , pharyngeal slits, dorsal nerve cord
describe urochordates. what is the garstang hypothesis?
• Exhibit all 5 chordate features in at least some developmental stage
Have a generalized body structure and lifestyle
• Garstang hypothesis - walter garstang - 1928
• Neotenic larval form that gave rise to the vertebrates
• Class Appendicularia has paedomorphic larvae - adults look like larvae
• However, adult sessile form is more derived condition
Predecessors of the vertebrates
describe cephalochordates. heart? kidney? liver? are they primitive?
• Have all 5 chordate features in adults
Have myomeres - v shaped vs. W shaped in vertebrates
• Circulator system with ventral heart
○ “heart” - simply an expanded blood vessel = sinus venosus
○ Not completely a closed system (more complex than an open system)
Have myocardial cells
• Have filter feeding similar to primitive type vertebrates
• Have hepatic caecum (= liver diverticulum)
○ Produce digestive enzymes
○ Resembles a sac in embryonic vertebrate that becomes a liver
• No kidney but have protonephridia (excretory tubules)
• Share solenocytes - specialized excretory cells
• Still not as advanced as vertebrates but not primitive
what evidence do we have for the origin of vertebrates?
fossils, hox genes, neural crest, micro RNA, brains
describe our fossil evidence
• Pikaia gracilens - mid cambrian 500 mya
• Haikouella - early cambrian 525 mya
○ A yunnanozoan - more derived
○ Notochord, myomeres, muscular pharynx, large brain, eyes, thickened branchial bars
○ Earliest chordates
describe our hox genes evidence
• Homeobox genes or hox gene complex
• Control the expression of other genes that direct the embryonic development of animals
• All animals have at least one hox gene
• Extant urochordates and cephalochordates each have a single hox cluster (approx. 12 genes)
• Vertebrates have undergone duplication of the entire Hox complex
○ Living jawless vertebrates have two clusters
○ All gnathostomes have at least 4 clusters
• Lead to structural complexity of vertebrates
describe our neural crest evidence
- A type of embryonic tissue unique to vertebrates - 4th germ layer is formed - helps with the structures listed below
- Cells are migratory and forms many structures, especially in the head region, including melanocytes, cranial cartilage and bone, nerves, some smooth muscle, etc.
- Similar type of migratory cells have been found in living urochordate larvae
describe our micro RNA evidence
- Non coding RNA sequences that regulate protein synthesis through mRNA binding
- Some microRNA are shared by vertebrates and urochordates
describe our brain evidence. there is a chart of all evidence in lecture 3 notes
• Amphioxus’ brain appears simple but contains almost all of the genes that code for the vertebrate brain - evidence of similarity
how many years did the first vertebrate evolve over? what did it evolve into?
• First vertebrate evolved over 540 mya
May have evolved into something like a soft bodied Conodonta and then into dermal-plated ostracoderm like Astraspis
