Lucy, officially named as Australopithecus afarensis is the name of pre-human or creature that lived 3.2 mil. years ago. It was founded in Ethiopia in Hadar region in 1974.

You can find very interesting data about her, but there is something that especially attracted my attention. Well, today we know that Lucy was oldest known human or pre-human individual that walked erect. What initiated that transition? Contemporary scientists think that there was a problem to find food in trees. Perhaps it was a good reason to think about descending to the ground. The ground was very dangerous because it was covered with high grass (savanna) and was populated with predators. Lucy was only 1.1 m (3 ft 7 in) tall and the grass was definitively taller than she was. That initiated the idea to stand erect to be able to see above that grass. Other reason was the fact that you spend less energy if you walk on two legs rather than on all four.
But how that influenced teeth development? It was not easy to stand upright because without any adaptation erected creature would look straight to the sky. To overcome that head should be flexed to the front and that was not so simple because in that case there would not be enough space for soft tissues which would in the and cut the airway. The evolution resolved that issue by descending the larynx. That opened much space in the posterior and enabled normal breathing and influenced development of speech.
That new space in the rear enabled also a great change in swallowing. Before that, every swallow included protrusion of the tongue between anterior teeth. That is the reason many animals have small incisors and/or open bite. With that additional space the tongue was able to go to the back and in the act of swallowing it’s tip was touching hard palate. The outcome was greater teeth in the anterior and closed bite for the first time.
Why the nature produced upper position of the larynx at all? Because it could be a dangerous and even deadly accident if food goes to the airway. That upper position of the larynx facilitated separation of digestive and respiratory systems. When erect body changed that separation wasn’t so effective like before and because of that we cannot drink water and breathe in the same time. In humans, that ancient type of swallowing is normally founded in babies, but before the end of the first year of life in normal development it changes to adult type of swallowing. If that does not occur, it is very likely that we will have open bite in the anterior.