No place on the planet has remained untouched by Homo sapiens, from the rainforests cleared for agriculture to the deep oceans polluted with microplastics to the altered jet streams caused by climate change. The world population reached 8 billion people in November. However, scientists now argue that our species could have easily never existed. Researchers in China have discovered evidence suggesting that 930,000 years ago, the ancestors of modern humans experienced a significant population crash, which they attribute to a drastic climate change that occurred around that time. During this bottleneck period, our ancestors were reduced to a population of fewer than 1,280 breeding individuals, threatening them with extinction. The population remained at low numbers for over 100,000 years before rebounding. The researchers suggest that this climate-driven bottleneck could have led to the split of early humans into two evolutionary lineages: one that gave rise to Neanderthals and another that gave rise to modern humans. However, experts are skeptical of the statistical methods used in the study. Despite this, scientists have been able to reconstruct the history of our species by analyzing the genes of living individuals. By comparing genetic variations in DNA, researchers can trace ancestry to ancient populations and infer population sizes at different points in history. The study conducted by Haipeng Li and his colleagues at the Chinese Academy of Sciences utilized a new method called FitCoal, which allows scientists to create a model of millions of years of evolution. They analyzed the genomes of 3,154 individuals from 50 populations around the world to explore different models and concluded that an extinction event occurred among our ancestors 930,000 years ago. The population declined from approximately 98,000 breeding individuals to fewer than 1,280 and remained small for 117,000 years before rebounding. This bottleneck hypothesis is consistent with the fossil record of human ancestors from that period. However, genetic diversity today could have resulted from alternative evolutionary histories, and more research is needed to test different models. Li and his colleagues propose that a global climate shift caused the population crash, as geological evidence shows that the Earth became colder and drier at the proposed timeframe. However, other experts argue that if a worldwide disaster occurred in Africa, it should have affected human relatives elsewhere as well, which is not evident from fossil records. Li and his colleagues also suggest that the population crash led to the split between modern humans and Neanderthals/Denisovans and the fusion of chromosomes, resulting in humans having 23 chromosomes instead of the typical 24 pairs in most apes. Overall, while the claim of a bottleneck is surprising, experts emphasize the need for stronger evidence to support this hypothesis.