Prior to the arrival of European explorers in the Americas in 1492, it is estimated that the population of the continent was around sixty million people. Over the next two centuries, most scholars agree that the indigenous population fell to just ten percent of its pre-colonization level, primarily due to the Old World diseases (namely smallpox) brought to the New World by Europeans and African slaves, as well as through violence and famine.

Distribution

It is thought that the most densely populated region of the Americas was in the fertile Mexican valley, home to over one third of the entire continent, including several Mesoamerican civilizations such as the Aztec empire. While the mid-estimate shows a population of over 21 million before European arrival, one estimate suggests that there were just 730,000 people of indigenous descent in Mexico in 1620, just one hundred years after Cortes' arrival. Estimates also suggest that the Andes, home to the Incas, was the second most-populous region in the Americas, while North America (in this case, the region north of the Rio Grande river) may have been the most sparsely populated region. There is some contention as to the size of the pre-Columbian populations in the Caribbean, as the mass genocides, forced relocation, and pandemics that followed in the early stages of Spanish colonization make it difficult to predict these numbers.

Varying estimates Estimating the indigenous populations of the Americas has proven to be a challenge and point of contention for modern historians. Totals from reputable sources range from 8.4 million people to 112.55 million, and while both of these totals were published in the 1930s and 1960s respectively, their continued citation proves the ambiguity surrounding this topic. European settlers' records from the 15th to 17th centuries have also created challenges, due to their unrealistic population predictions and inaccurate methodologies (for example, many early settlers only counted the number of warriors in each civilization). Nonetheless, most modern historians use figures close to those given in the "Middle estimate" shown here, with similar distributions by region.

Following the arrival of Spanish colonizers in 1519, namely Hernando Cortes and his 600 conquistadors, the indigenous population of the Mexican valley saw a dramatic decline. In the first two years of conquest, thousands of indigenous Americans perished while fighting the European invaders, including an estimated 100,000 who died of violence or starvation during Cortes' siege of the Aztec capital city, Tenochtitlan (present-day Mexico City), in 1520. However, the impact of European violence on population decline pales in comparison to the impact of Old World diseases, which saw the indigenous population of the region drop from roughly 22 million to less than two million within eight decades.. Virgin soil pandemics Almost immediately after the Spanish arrival, a wave of smallpox swept across the indigenous populations, with some estimates suggesting that five to eight million natives died in the subsequent pandemic between 1519 and 1520. This outbreak was not an isolated incident, with the entire indigenous population of the Americas dropping by roughly ninety percent in the next two centuries. The Mexican valley specifically, which was the most populous region of the pre-Columbian Americas, suffered greatly due to virgin soil pandemics (where new diseases are introduced to biologically defenseless populations). In the Middle Ages, the majority of Europeans contracted smallpox as children, which generally granted lifelong immunity. In contrast, indigenous Americans had never been exposed to these diseases, and their populations (of all ages) declined rapidly. Cocoliztli Roughly three decades after the smallpox pandemic, another pandemic swept across the valley, to a more devastating effect. This was an outbreak of cocoliztli, which almost wiped out the entire population, and was followed by a second pandemic three decades later. Until recently, historians were still unsure of the exact causes of cocoliztli, with most hypothesizing that it was a rodent-borne disease similar to plague or an extreme form of a haemorrhagic fever. In 2018, however, scientists in Jena, Germany, studied 29 sets of teeth from 16th century skeletons found in the Oaxaca region of Mexico (from a cemetery with known links to the 1545 pandemic); these tests concluded that cocoliztli was most likely an extreme and rare form of the salmonella bacterium, which caused paratyphoid fever. These pandemics coincided with some of the most extreme droughts ever recorded in North America, which exacerbates the spread and symptoms of this disease, and the symptoms described in historical texts give further credence to the claim that cocoliztli was caused by salmonella.

This study describes the genetic diversity and population structure of 194 native maize populations from 23 countries of Latin America and the Caribbean. The germplasm, representing 131 distinct landraces, was genetically characterized as population bulks using 28 SSR markers. Three main groups of maize germplasm were identified. The first, the Mexico and Southern Andes group, highlights the Pre-Columbian and modern exchange of germplasm between North and South America. The second group, Mesoamerica lowland, supports the hypothesis that two separate human migration events could have contributed to Caribbean maize germplasm. The third, the Andean group, displayed early introduction of maize into the Andes, with little mixing since then, other than a regional interchange zone active in the past. Events and activities in the pre- and post-Columbian Americas including the development and expansion of pre-Columbian cultures and the arrival of Europeans to the Americas are discussed in relation to the history of maize migration from its point of domestication in Mesoamerica to South America and the Caribbean through sea and land routes.