Traces early animal evolution before true vertebrates, from the notochord and cartilage-like feeding supports of Amphioxus to the centralised neural tube and single-layered skin of early chordates. It then covers the origin of the head and jaws, the 2R genome duplication hypothesis, and the jawless agnathans, closing with the arrival of bone in placoderms. Four mnemonics summarise the key innovations, the early chordate features, the genome duplication consequences and the pre-vertebrate traits.
Inside: 11 sections · 10 flashcards · 7 min
Topic 61 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 11 sections, about 898 words (7 min of reading), with 10 flashcards.
In this note
Animal evolution before vertebrates
The Earliest Skeletal Structure: The Notochord
Early Endoskeletal Elements in Primitive Chordates
Bone is Unique to Vertebrates
Evolution of the Nervous System
Early Chordate Skin Structure
Evolution of the Head and Feeding Structures
Role of Genome Duplications in Vertebrate Evolution
The Earliest Vertebrates: Jawless Fishes (Agnathans)
Key Evolutionary Milestones After Agnathans
MNEMONICS
The Earliest Skeletal Structure: The Notochord
The notochord is considered the earliest primitive axial skeletal structure in vertebrate evolution.
It gives the phylum Chordata its name and is essential during early development.
In adult vertebrates, it is present only in a few species as it is typically replaced by the vertebral column.
Anatomically, the notochord lies below the neural tube and above the digestive system.
It provides structural support and plays a central role in cell signaling and development during the embryo's early stages through embryonic induction.
Early Endoskeletal Elements in Primitive Chordates
In primitive chordates such as Branchiostoma (also called Amphioxus), parts of the feeding apparatus are made of cartilage-like tissue.
This cartilage is acellular, yet stiff and made up of a matrix that includes fibrillar collagen, forming the pharyngeal endoskeleton.
Cartilage in these organisms is non-mineralized, but highly resilient.
It consists of chondrocytes suspended in an extracellular matrix rich in collagen and glycosaminoglycans.
The development of cartilage involves transcription factors such as SoxD and SoxE.
Primitive forms of cartilage and chondrocyte-like cells are found across many bilaterians, not just in vertebrates. This suggests these features may have existed in a common bilaterian ancestor.
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