Human Phylogeny and Ontogeny

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.

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, 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: monotremes (egg-laying
mammals), marsupials (mammals with pouches), and placentals (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


B Different stages in the early embryonic development of
The early developmental stages (top row) of fish, amphibians, reptiles,
birds, and mammals (as represented by humans) presentaseries of
striking similarities that suggest a common evolutionary origin. One
particularly noteworthy common feature is the set of branchial or pharyngeal arches in the embryonic regions that will develop into head and
neck. Although it was once thought that the developing embryo of a
specific vertebrate would sequentially display features from organisms
representing every previous step in its evolution (“ontogeny recapitulates phylogeny”, the “biogenetic law” of Ernst Haeckel (1834–1919)),
subsequent work has shown that the vertebrates share common embryonic components that have been adapted to produce sometimes
similar (fins and limbs) and sometimes radically different (gills vs. neck
cartilages) adult structures.

C Formation of the branchial or pharyngeal arches in a five-weekold human embryo
Left lateral view. The branchial or pharyngeal arches of the vertebrate
embryo have a metameric arrangement (similar to the somites, the
primitive segments of the embryonic mesoderm); this means that they
are organized into a series of segments that have the same basic structure. Among their other functions, they provide the raw material for the
species-specific development of the visceral skeleton (maxilla and mandible, middle ear, hyoid bone, larynx), the associated facial muscles, and
the pharyngeal gut

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