The question of whether the anus is the first anatomical feature to develop in a human embryo often appears in discussions about biology and evolution. This concept has a basis in scientific observation, but the full answer requires understanding the precise stages of human development. The initial single cell rapidly transforms through cellular divisions and rearrangements, which establishes the body plan and determines the fate of the two ends of the digestive tract.
The Direct Answer: Defining Deuterostomes
The claim that the anus develops before the mouth is rooted in a fundamental classification system used to categorize the animal kingdom. Animals with bilateral symmetry are divided into two major groups: protostomes and deuterostomes. This distinction is based on the fate of the first opening to form in the developing embryo, known as the blastopore.
The term “protostome” translates to “first mouth,” meaning the blastopore develops into the mouth, with the anus forming secondarily. This pattern is seen in many invertebrates, such as insects, worms, and mollusks. Conversely, the term “deuterostome” means “second mouth,” indicating a different sequence.
Humans, along with all other vertebrates, are classified as deuterostomes. In this group, the blastopore forms the opening that will eventually become the anus, while the mouth develops later as a secondary opening. This developmental pattern confirms that the precursor to the anus is established before the precursor to the mouth.
The Absolute Earliest Processes
While the concept of anus-first development defines our evolutionary lineage, the anus is not the absolute first structure to form in a human embryo. Development begins immediately after fertilization with rapid cell division called cleavage. The single-celled zygote divides repeatedly without increasing in size, resulting in a cluster of smaller cells called blastomeres.
By about day four, the cluster forms a solid ball known as the morula. Fluid accumulates inside this mass, transforming it into a hollow blastocyst around day five. The blastocyst consists of an inner cell mass, which becomes the embryo, and an outer layer that contributes to the placenta. The blastocyst implants into the uterine wall between day six and day ten. These early cellular stages occur well before the anus precursor appears.
Gastrulation and the Blastopore
The formation of the structure that will become the anus occurs during gastrulation. This cellular rearrangement transforms the two-layered blastocyst into a three-layered structure, establishing the basic body plan. Gastrulation begins around the third week after fertilization, approximately day 17.
The first visible sign is the formation of the primitive streak, a thickened linear structure on the surface of the embryonic disc. Cells migrate inward through the streak, establishing the three primary germ layers: the ectoderm, mesoderm, and endoderm. The endoderm forms the lining of the digestive and respiratory systems, the mesoderm forms muscle, bone, and the circulatory system, and the ectoderm gives rise to the nervous system and skin.
The caudal end of the primitive streak is the location that ultimately gives rise to the opening that forms the anus. This region is where the future hindgut terminates, establishing the anus-end of the gut tube before the mouth-end.
Comparative Timing of Key Organ Development
To fully address the claim of the anus being the “first” structure, its precursor’s timing must be compared with other major organ systems that emerge simultaneously. While the blastopore precursor is present during gastrulation in the third week, other processes begin almost concurrently.
The development of the nervous system, known as neurulation, starts shortly after gastrulation. Neurulation involves the folding of the ectoderm into the neural tube, the foundation for the brain and spinal cord. The neural plate begins to form as gastrulation progresses, with the neural tube closing between day 24 (cranial end) and day 28 (caudal end).
The circulatory system also begins formation very early. Precursors to the heart tube appear as early as day 19, fusing into a single primitive heart tube by day 21. This early heart structure is capable of electrical conduction and contraction, making the heart the first organ to begin functioning.
Thus, the blastopore precursor is not an isolated first event. The establishment of the gut axis, the nervous system, and the primitive cardiovascular system are overlapping processes that begin almost simultaneously during the third and fourth weeks. However, the mouth, which develops from a separate opening called the stomodeum, is definitively formed later than the anus precursor, supporting the “anus-first” concept for the digestive tract’s two ends.