Hammer developed her fascination with moss mites as a zoology student when she read the zoologist Carl Heinrich Bornebusch’s (1886–1951) work on the fauna of forest soil. She learnt that these barely visible arthropods play a key role in breaking down organic material and thus in the earth’s ecology. This encounter with an almost invisible world inspired a life-long research endeavour.

From the 1930s onwards, Hammer went on independent research trips to Greenland, Iceland, Norway, Iran, Indonesia, New Zealand, Canada and large parts of South America to collect soil samples. The work was back-breaking and exhausting, but she was driven by a love of travel and professional persistence.

With her Berlese funnel, named after the Italian entomologist Antonio Berlese (1863–1927), she collected moss mites from a diverse range of landscapes, from the arctic tundra to humid rainforests. The funnel was based on a method developed by Berlese during the early 20th century for the systematic extraction of tiny creatures from soil.

Back in her sitting room, which served as her laboratory, she sorted and described thousands of moss mites. Viewing them under the microscope, she made drawings of their structures, identified new species and traced their ancestry.

Ultimately, she described more than 1,500 species and many new genera, and her work was acknowledged as an exceptional accomplishment in soil ecology and systematisation. Despite international recognition as a leading expert on moss mites, she never achieved tenure at a Danish research institution – something she said was ‘a bummer’.

However, Hammer’s research was not just remarkable for the many descriptions of species. As early as her doctoral dissertation from 1944, she was documenting striking similarities between the microfauna of Greenland and species found in North America and Europe. The same patterns emerged when she worked in the arctic region of Canada, during the late 1940s, and compared her finds with collections at the Smithsonian Institution and Harvard University.

Gradually, the outline of a scientific mystery began to take shape: moss mites barely move, do not fly and cannot survive in salt water – so how was it possible that such closely related species existed on continents thousands of kilometres apart?

Hammer discovered the answer through what she later called ‘a miracle’. On an expedition to New Zealand in 1962–1963, she found the species Mucronothrus nasalis, which she had previously seen in Greenland, Europe and the Andes. The puzzle’s pieces fell into place: the only plausible explanation was that the continents had once been connected – exactly as the geologist Alfred Wegener (1880–1930) had suggested.

In 1965, she published her most significant paper, arguing that the moss mites’ global dispersion patterns reflect a time when the earth looked very different. Her work was met with both curiosity and scepticism, but she found support from other scientists, including the entomologist John Anthony Wallwork (1932–2004), and her findings became an early biological parallel to the emerging theory on plate tectonics.

Today, Marie Hammer’s research represents a remarkable bridge between biology and geology. Her work showed how even the smallest organisms can carry traces of the biggest events in Earth’s history. As she wrote in her autobiography, she was never happier than when she was observing her ‘tiny world’ through the lens of a microscope.

The chapter is written by Kristian Sjøgren.