Swidden agriculture — often called “slash-and-burn” — has been practiced on every inhabited continent throughout human history, and for decades it has been blamed for the destruction of the world’s rainforests. However, a new study of more than 18,000 forest patches around the world found that the environmental impact of swidden is more nuanced, and less destructive, than the term “slash-and-burn” suggests — and that unwritten social rules in small societies, not government oversight, often regulate land use closely enough to support both harvests and forest regeneration sustainably over time.

Before entering a long decline that continues into the present, humanity may have spoken far more languages, experiencing a “golden age” of linguistic diversity, with tens of thousands of languages spoken worldwide only several thousand years ago, according to a new study. Human language is extraordinarily diverse, with roughly 7,500 signed and spoken in the world today. However, this diversity has been steadily eroding for centuries as a small number of dominant languages have spread worldwide. Today, nearly half of all languages are endangered, and at least four are lost each year. Because written records extend back only about 6,000 years, the long-term patterns of linguistic diversity across much of human history remain largely unknown. To reconstruct the history of the world's languages over the past 12,000 years, Damián Blasi and colleagues developed a demographic model using ethnographic data from 171 hunter-gatherer societies as a proxy for early human populations. Because these small, culturally cohesive groups typically spoke a single language, the model used their sizes, together with estimates of ancient global population, to infer the likely number of languages in the early Holocene. The model assumes that as agriculture and technological advances allowed populations to grow, increasing numbers of people came to share the same language, causing language diversity to grow more slowly than the human population.  

According to Blasi et al., the approach provides a quantitative framework for reconstructing the long-term history of the world's languages despite the lack of direct historical evidence. The findings estimate that the world likely had fewer languages 12,000 years ago than it does today, contradicting the idea that linguistic diversity has steadily declined since the end of the last Ice Age. Instead, the number of languages appears to have increased alongside growing human populations, peaking between 1,000 and 3,000 years ago, when the world may have supported tens of thousands of languages. The model suggests that this "golden age" of linguistic diversity was followed by an exceptionally rapid collapse over the past two millennia, as expanding societies increasingly replaced smaller language communities. According to the authors, the findings indicate that today's language crisis is the culmination of a long-term process of cultural homogenization driven by the expansion of human population rather than a consequence of colonialism alone.