22.7 Billion chickens — that’s how many broiler chickens are being raised for food at any given time, making them far and away the most numerous bird on the planet. And it’s not just the size of the poultry population that’s remarkable. Humans have bred today’s chickens to grow so much meat so quickly that they couldn’t survive to adulthood in the wild. A new British-led study, published in the journal Royal Society Open Science, puts forth the broiler chicken as a distinct marker of the Anthropocene, a proposed geological epoch marked by humankind’s transformation of the earth. “We think they are a really important symbol and potential future fossil of this age, and man’s impact on the planet,” researcher Carys Bennett of the University of Leicester told the publication New Scientist. The 60 billion chickens that are slaughtered for food every year leave their bones behind. After being tossed into landfills and buried under other trash, the bones could remain for millions of years.