Developing vertebrate embryos share so many features that it is often difficult to tell one species from another. From a single cell, the development of the fertilized egg, or zygote, begins as a single cell, and through multiple cell divisions, proceeds along a familiar path. Regardless of the species, during early development, the same groups of cells develop in the same order and in similar patterns, producing tissues and organs that are common to all. This similarity illustrates that the many processes guiding embryonic development have been preserved by evolution, and replayed time and time again. It also means that scientists interested in understanding human development can learn by observing the development of other organisms that are easier to study.

Just hours into development, a typical vertebrate embryo is already made up of thousands of dividing cells, called stem cells, which are arranged into a dense ball called a blastula. At this stage, each stem cell is only one-fourth to one-tenth the diameter of the original zygote but is otherwise nearly identical to it. The majority of organisms, however, have many more than one type of cell. Humans have approximately 200 different types, each with a specific form and function. The process by which stem cells give rise to such variety is one of the most important aspects of development.

Stem cells begin their transformation into the different types of cells that make up the human body through a process called cell differentiation. In vertebrates, this process takes place during a stage called gastrulation, when distinct tissue layers first form. Like most other developmental processes, differentiation is controlled by genes—the genetic instructions encoded in the DNA of every cell. Genes instruct each cell how and when to build the proteins that allow it to create the structures, and ultimately perform the functions, specific to its type of cell.

Given that most cells contain identical sets of genes, cell differentiation requires a great deal of coordination. Depending on the type of cell required, certain genes must be activated during this process, while others remain inactive. This is typically dictated by the placement of a cell in the gastrula, but the job of turning genes on and off in various cells belongs to a relatively small number of so-called master control genes. Not only do these genes dictate the role of individual cells, but they also determine the body plan for the organism as a whole.

In addition, the fact that a wide variety of organisms share identical master control genes explains why these organisms follow similar developmental paths.

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