Where is the line that divides the single continental mass of America into North and South?

Geographical
The border between North America and South America is primarily geographical in nature. This geographical definition is largely defended by pointing out that the entirety of North America lies within the Northern Hemisphere. This land mass is largely surrounding by water, with the Pacific Ocean located to the west, the Atlantic Ocean located to the east, the Arctic Ocean located to the north, and the Caribbean Sea located to the southeast. The continent of South America is located to its far southeast, connected only by the Isthmus of Panama. North America covers a total area of approximately 9.54 million square miles and has a total population of around 565 million individuals. These numbers represent about 16.5% of the total land area on earth and around 7.5% of its total human population.

South America, in contrast, is located almost entirely within the Southern Hemisphere. Like North America, South America is largely surrounding by water, with the Pacific Ocean located to the west and south, the Atlantic Ocean located to the east, and the Caribbean Sea located to the northeast. North America is located to its far northwest. South America covers a total area of approximately 6.89 million square miles and has a total population size of around 420 million.

The geographic boundary between North America and South America is, therefore, defined as the Isthmus of Panama. This stretch of land measures between 30 miles wide at its narrowest point and 120 miles at its widest point. Within this isthmus, the divide between the north and south is generally agreed to be the Darien Gap. This region prevents the two continents from being linked by the Pan-American highway. Construction of this highway has not yet been completed through the Darien Gap due to its large area of swamps, waterways, and forests that make development difficult. Additionally, international agencies have worked to prevent its construction, citing extensive environmental damage as a side effect.