General Anatomy and Musculoskeletal System (THIEME Atlas of Anatomy), Latin Nomenclature (eBook)

1 Human Phylogeny and Ontogeny

1.1Human Phylogeny

A Brief overview of human phylogenetic development

To better understand the evolution of the human body, it is helpful to trace its phylogenetic development. Humans and their closest relatives belong to the phylum Chordata, which includes approximately 50,000 species. It consists of two subphyla:

• Invertebrata: the tunicates (Tunicata) and chordates without a true skull (Acraniata or Cephalochordata)

• Vertebrata: the vertebrates (animals that have a vertebral column)

Although some members of the chordate phylum differ markedly from one another in appearance, they are distinguished from all other animals by characteristic morphological structures that are present at some time during the life of the animal, if only during embryonic development (see G). Invertebrate chordates, such as the cephalochordates and their best-known species, the lancelet (Branchiostoma lanceolatum) are considered the model of a primitive vertebrate by virtue of their organization. They provide clues to the basic structure of the vertebrate body and thus are important in understanding the general organization of vertebrate organisms (see D).

All the members of present-day vertebrate classes (jawless fish, cartilaginous fish, bony fish, amphibians, reptiles, birds, and mammals) have a number of characteristic features in common (see H), including a row of vertebrae arranged in a vertebral column (columna vertebralis), which gives the subphylum its name (Vertebrata). The evolution of an amniotic egg, i.e. the development of the embryo within a fixed shell inside a fluid-filled amniotic cavity, was a critical evolutionary breakthrough that helped the vertebrates to survive on land. This reproductive adaptation enabled the terrestrial vertebrates (reptiles, birds, and mammals) to live out their life cycles entirely on land and sever the final ties with their marine origin. When we compare the embryos of different vertebrate classes, we observe a number of morphological and functional similarities, including the formation of branchial arches (see B).

Mammals comprise three major groups: Monotremata (egg-laying mammals), Marsupialia (mammals with pouches), and Placentalia (mammals with a placenta). The placental mammals, which include humans, have a number of characteristic features (see I), including a tendency to invest much greater energy in the care and rearing of their young. Placental mammals complete their embryonic development inside the uterus and are connected to the mother by a placenta. Humans belong to the mammalian order of primates, whose earliest members were presumably small tree-dwelling mammals. Together with lemurs, monkeys, and the higher apes, human beings have features that originate from the early adaptation to an arboreal way of life. For example, primates have movable shoulder joints that enable them to climb in a hanging position while swinging from branch to branch. They have dexterous hands for grasping branches and manipulating food, and they have binocular, broadly overlapping visual fields for excellent depth perception.

G Characteristic features of chordates

H Characteristic features of vertebrates

I Characteristic features of mammals

1.2Human Ontogeny: Overview, Fertilization, and Earliest Developmental Stages

Besides gross and microscopic anatomy, the developmental history of the individual organism (ontogeny) is of key importance in understanding the human body. Ontogeny is concerned with the formation of tissues (histogenesis), organs (organogenesis), and the shape of the body (morphogenesis).