Figure 2. Evidence supportive of adult hippocampal neurogenesis in humans.

Black line shows atmospheric concentration of ¹⁴C by year; the spike reflects above-ground nuclear bomb tests between 1945 and 1963. Blue dots reflect hippocampal ¹⁴C concentrations from postmortem tissue, plotted by birth date. The presence of dots above the line for individuals born before the spike, but below the line for individuals born after it, strongly suggests adult hippocampal neurogenesis. Humans take up ¹⁴C from the plants and animals they eat, and the ¹⁴C is incorporated into the DNA when the cells in the body divide to form new cells (neurons). Thus, the fact that adults born before the spike have higher than expected hippocampal ¹⁴C concentrations suggests that new hippocampal neurons were added later in their lives, when atmospheric concentrations of ¹⁴C were elevated. By the same token, the fact that adults born after the spike have lower than expected levels is consistent with the hypothesis that hippocampal neurons were added in adulthood as ¹⁴C concentrations fell. Image adapted from [18], with permission.