Boning up on early skeletons

Palaeontologists have identified the oldest known form of bone, solving a 160-year-old mystery about the evolution of the human skeleton.

Led by Joseph Keating from the University of Manchester in the UK, a team of researchers used high-energy X-rays to examine the fossilised skeletons of one of our oldest vertebrate relatives: ancient fish called heterostracans.

The fish skeletons are made of aspidin, a tissue with a structure of crisscrossing tubes. Unlike anything found in modern vertebrates, it was thought to be a precursor to bones as we know them today.

“For 160 years, scientists have wondered if aspidin is a transitional stage in the evolution of mineralised tissues,” explains Keating.

Now, new findings published in the journal Nature Ecology and Evolution reveal that the tiny tubes are in fact openings that formerly held bundles of collagen, a type of protein found in skin and – critically – bones.

The discovery places the origin of the vertebrate skeleton at an earlier date than previously assumed.

“We show that [aspidin] is, in fact, a type of bone, and that all these tissues must have evolved millions of years earlier,” says co-author Phil Donoghue from the University of Bristol, UK.

Heterostracans are an extinct group of jawless fish that inhabited salt and fresh water habitats during the early to middle Palaeozoic era some 440 to 359 million years ago. In comparison, flowering plants appear in the fossil record around 140 million years ago, and modern humans just 200,000 years ago.

“These findings change our view on the evolution of the skeleton,” says Donoghue.