The demise of big animals in the Amazon region 12,000 years ago cut a key way that nutrients were distributed across the landscape, a study has suggested.
Researchers say animals such as huge armadillo-like creatures would have distributed vital nutrients for plants via their dung and bodies.
The effects, still visible today, raise questions about the impact of losing large modern species like elephants.
The findings have been published in the journal Nature Geoscience.
A team of UK and US researchers developed a mathematical model to calculate what impact the sudden loss of megafauna - animals with a body mass of more than 44kg (97lb) - had on the Amazonia's ecosystem.
Results showed that the extinctions resulted in a 98% reduction in the dispersal of phosphorus (chemical symbol "P").
"This resulted in strong decreases in phosphorus availability in eastern Amazonia away from fertile floodplains, a decline that may still be ongoing," the scientists wrote.
"The current P limitation in the Amazon basin may be partially a relic of an ecosystem without the functional connectivity it once had."
In mammals, phosphorus is the second most abundant mineral in the body; calcium is the most abundant.
The vast majority is found in bones and teeth, but the nutrient also plays a vital role in functions such as filtering out waste in the kidneys, and how bodies store and uses energy.
Essential for life
Phosphorus is also a key mineral for plants. It is present in every cell and is vital, as it is in mammals.
There are a number of theories to explain the terminal demise of many of the planet's largest animals about 12,000 years ago.
These include increased hunting pressure as a result of the spread of early humans, climate change, the spread and evolution of diseases or the impact and aftermath of a massive space rock hitting the Earth.
Co-author Adam Wolf, from Princeton University's Department of Ecology and Evolutionary Biology, explained: "On today's planet, the supply of nutrients in the soil is determined by river deposits or nutrients that are airborne."
But Dr Wolf added that their study suggested that things were once very different: "We believe that large animals once played a vital role in fertilising their landscape, so that the naturally occurring deposits in rock were less important.
This was the result of the megafauna's high rates of food consumption, combined with their "large daily ranges and their long gut residence times".
"If humans contributed to the mass extinction of big animals 12,000 years ago, this suggests that humans started to affect the environment at global scales before the dawn of agriculture," Dr Wolf observed.
The team estimated that following the mass extinction, the ranges of animals, and therefore the distribution of nutrients, fell from a mean distance of 61.8 km (38.4 miles) to 4.8 km (3.0 miles). They also calculated that the distance between eating and excretion fell from 9.1km to 2.1km.
The team said the study was the first of its kinds to look at the impact of the mass extinction on ecosystem nutrient biogeochemistry through the transport of dung and bodies.
Commenting on the findings, lead author Christopher Doughty, from the University of Oxford's Environmental Change Institute, said: "Put simply, the bigger the animal, the bigger its role in distributing nutrients that enrich the environment.
"Most of the planet's large animals have already become extinct, thereby severing the arteries that carried nutrients far beyond the rivers into infertile areas.
"We can also predict the effects of further extinctions - a fate fast approaching many of the large animals that remain - and examine the likely impact thousands of years into the future."