Why have Louis Agassiz's claims about a racial hierarchy been rejected by science?

Louis Agassiz’s claims regarding a racial hierarchy and polygenism—the belief that different human races were created independently as separate species—have been rejected by modern science because they are fundamentally unfounded and lack biological validity.123

Agassiz’s views were part of the scientific racism prevalent in the 19th century, which sought to justify social stratification through pseudoscientific hypotheses. These claims have been debunked by subsequent anthropological and genetic research for several reasons:14

• Lack of Biological Basis: Modern DNA analysis demonstrates that all humans share more than 99 percent of their genetic material. There are no genes that identify distinct groups corresponding to conventional racial categories.5
• Overlapping Variation: Human physical traits, such as skin color and hair texture, vary in subtle gradations across geographic space rather than falling into discrete, immutable packages.567
• Internal Diversity: Genetic studies have shown that internal variations within any given population are often greater than the differences between populations.567
• Social Construction: Scholars now recognize that race is a social invention rather than a biological reality, with no scientific consensus ever existing on the number or definition of races.5