If you did not know about the life of the Buddha, would you be able to tell what the sculpture depicts?
No, without knowing the life of the Buddha, it would not be possible to tell what the sculpture depicts.
Early Buddhist sculptors did not show the Buddha in human form. Instead, they showed his presence through symbols like the empty seat (indicating meditation), the stupa (representing mahaparinibbana), and the wheel (standing for the first sermon at Sarnath).
Such sculptures cannot be understood literally. For instance, a tree does not simply stand for a tree, but symbolises an event in the life of the Buddha. To understand these symbols, art historians had to acquire familiarity with hagiographies and the traditions of those who produced these works of art.
Explanation
The textbook clearly explains that early Buddhist art was aniconic - the Buddha was represented through symbols rather than human form. Without knowledge of Buddha's life events (enlightenment under the tree, first sermon at Sarnath, mahaparinibbana), a viewer would see only literal objects (tree, wheel, stupa) and miss their symbolic significance. This is why art historians needed to study hagiographies to interpret these sculptures correctly.