If in an eight-cell embryo of an organism, the cells of the upper tier are aligned over the cells of the lower tier at a slight angle, it is likely that the organism is a vertebrate.
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A cell is analogous to an engineered watch because it consists of numerous interdependent sub units and also because coded information can not arise from random processes. A cell is the basic unit of living systems. Although it is relatively easy to visualize the components of cells, it is difficult to conceptualize how these components function together to sustain life within the cell.
Natural selection provides the outline of an explanatory theory." Biologists consider it to be a scientific fact that evolution has occurred in that modern organisms differ from past forms, and evolution is still occurring with discernible differences between organisms and their descendants.