Before sunrise, a sunflower looks east in anticipation of the coming dawn. Then, the flower’s face tracks the sun from east to west until it dips below the horizon. Overnight, the flower head slowly but deliberately orients back to the east to start again. How do sunflowers accomplish this solar tracking? During the day, one side of the stem grows, tilting the flower on its solar trajectory. At night, the other side of the stem elongates, angling the flower back to its starting position. This rhythm evolved because it provided growing flowers with maximal sun exposure, and the flowers warmed in the sun attracted more pollinators. Sunflowers responding to patterns of light and dark reveal the utility—and the power—of an organism syncing with the daily cycles of the Earth.
When we think of how life evolved on Earth, we usually only think about the three-dimensional environments that provided the selection pressure for the evolution of organisms. However, the recurring nature of day and night and the changing levels of light throughout the seasons were prominent features of our planet even before life arose, so organisms evolved in response to these temporal rhythms as well. For example, most plant species are diurnal. Their flowers bloom in the day and close at night, which makes sense because of photosynthesis. The flowers of nocturnal plants are closed in the day and bloom at night because they evolved to take advantage of nocturnal pollinators, such as moths and bats. The earliest mammals were nocturnal, because that temporal niche initially enabled them to survive and thrive in the age of dinosaurs. Most current mammals are still nocturnal, but independent evolutionary transitions to a diurnal pattern have occurred. For example, the great apes evolved to be active during the day. Conversely, most species of birds are diurnal, although a few, such as owls, made the nocturnal evolutionary transition to take advantage of nighttime prey. Nocturnal or diurnal, all plants and animals evolved to fill all of nature’s spatial and temporal niches.
All life has the internal ability to keep time, because hidden inside every cell, a clock evolved to respond to the cycles of our planet (figure P1.1). We call these timekeepers circadian rhythms, named for the word circadian, which means “around the day.” These rhythms govern almost every aspect of an organism’s life, including gene expression, hormone secretion, even patterns of activity and rest. These rhythms not only allow an organism to react to a stimulus, such as a sunflower following the light of the sun, but they also allow an organism to anticipate a change in the environment, such as a sunflower reorienting itself at night to be ready for the dawn.