Sexually transmitted infections (STIs) are caused by pathogens requiring intimate human contact for transmission, making their history inseparable from human society. Tracing the first appearance of these pathogens involves a complex investigation across biological, archaeological, and historical records. Their emergence is not a single point but a spectrum, ranging from ancient infections that co-evolved with early hominids to modern diseases resulting from recent animal-to-human transmission events.
Methods for Tracing Disease Origins
Determining the deep history of a pathogen relies on three primary lines of evidence: paleopathology, historical documentation, and molecular dating.
Paleopathology
Paleopathology involves studying ancient human remains, primarily skeletons and teeth, to find physical evidence of disease. This method is limited because only a few infections, such as those caused by Treponema pallidum, leave distinctive, visible markers on bone tissue.
Historical Documentation
Historical medical texts from civilizations like ancient Egypt, Greece, and Rome describe symptoms such as genital ulcers and discharges. While helpful for understanding when certain symptoms became prevalent, the lack of modern diagnostic precision means these accounts cannot definitively pinpoint the origin of a specific microbe.
Molecular Dating
The most definitive evidence comes from paleogenomics, which uses molecular clock analysis on ancient pathogen DNA. Scientists sequence microbial DNA from archaeological specimens and compare it with modern strains to estimate when the strains diverged from a common ancestor. This technique calculates the pathogen’s evolutionary rate, establishing a time of most recent common ancestor (TMRCA) for the infection.
Infections Rooted in Ancient Human History
Some STIs stretch back hundreds of thousands of years, suggesting deep integration into the human evolutionary narrative. Genital Herpes, caused by Herpes Simplex Virus type 2 (HSV-2), is one example. Genetic evidence suggests the virus began infecting human ancestors over 1.5 million years ago, long before the emergence of modern Homo sapiens.
This ancient origin indicates that HSV-2 likely jumped from early hominids, and its initial transmission was not strictly venereal. As human social structures evolved, the virus adapted to venereal transmission as the most efficient means of spread.
Gonorrhea also appears in the earliest records of human disease. The Greek physician Galen coined the term “gonorrhea” around 130 A.D. Earlier descriptions of urethral discharge, possibly corresponding to the infection, appear in the writings of Hippocrates (400 B.C.) and in Egyptian papyri (1550 B.C.). Since the bacterium Neisseria gonorrhoeae does not leave skeletal traces, its deep history relies heavily on genetic analysis and interpretation of these historical descriptions.
The Global Appearance of Syphilis
The most dramatic emergence of an STI is the sudden appearance and rapid spread of venereal syphilis in Europe, referred to as the “Great Pox.” The disease swept the continent starting in the 1490s, shortly after Christopher Columbus’s return from the Americas. This timing led to the long-standing “Columbian Hypothesis,” which posits that the virulent venereal form of the disease was brought back by his crew.
The bacterium responsible is Treponema pallidum subspecies pallidum. Competing theories suggested the infection was already present in Europe but was a milder, non-venereal form that mutated. Paleopathological discoveries of syphilis-like lesions on pre-Columbian European skeletons fueled this “pre-Columbian” debate.
Recent ancient DNA analysis has complicated this picture. Genetic material from Treponema pallidum has been recovered from pre-Columbian remains in the Americas, dating back 2,000 years. This confirms a deep history of treponematoses, the family of diseases including syphilis, in the New World. The current consensus acknowledges diverse non-venereal treponemal infections existed globally, but the specific, highly virulent venereal syphilis that caused the 15th-century European epidemic appears to have originated in the Americas and spread rapidly following transatlantic voyages.
20th Century Zoonotic Jumps
Infections emerging in modern history often result from recent zoonotic transmission events, where a pathogen jumps from an animal host to a human one. The most globally significant example is the Human Immunodeficiency Virus (HIV), the cause of AIDS. HIV is directly traced back to the Simian Immunodeficiency Virus (SIV) that naturally infects African primates, such as chimpanzees and sooty mangabeys.
Multiple cross-species transmissions occurred, leading to different groups of HIV, but the strain responsible for the global pandemic, HIV-1 Group M, jumped from chimpanzees (SIVcpz). Molecular clock analysis estimates that the most recent common ancestor for HIV-1 Group M began spreading in the human population around 1931.
The initial transmission and subsequent epidemic spread are linked to the growing urban center of Léopoldville, now Kinshasa, in the Democratic Republic of Congo, around the 1920s. The virus gained a foothold and expanded due to factors like population growth, changing social dynamics, and the development of transportation infrastructure that facilitated its movement out of Central Africa. Although the virus spread silently for decades, the world did not recognize the new disease until the early 1980s, marking the start of the global AIDS pandemic.