Interspecies competition led to even more forms of ancient human -- defying evolutionary trends in vertebrates

Competition between species played a major role in the rise and fall of hominins -- and produced a "bizarre" evolutionary pattern for the Homo lineage -- according to a new University of Cambridge study that revises the start and end dates for many of our early ancestors.

Conventionally, climate is held responsible for the emergence and extinction of hominin species. In most vertebrates, however, interspecies competition is known to play an important role.

Now, research shows for the first time that competition was fundamental to "speciation" -- the rate at which new species emerge -- across five million years of hominin evolution.

The study, published today in Nature Ecology & Evolution, also suggests that the species formation pattern of our own lineage was unlike almost anything else.

"We have been ignoring the way competition between species has shaped our own evolutionary tree," said lead author Dr Laura van Holstein, a University of Cambridge biological anthropologist from Clare College. "The effect of climate on hominin species is only part of the story."

In other vertebrates, species form to fill ecological "niches" says van Holstein. Take Darwin's finches: some evolved large beaks for nut-cracking, while others evolved small beaks for feeding on certain insects. When each resource niche gets filled, competition kicks in, so no new finches emerge and extinctions take over.

Van Holstein used Bayesian modelling and phylogenetic analyses to show that, like other vertebrates, most hominin species formed when competition for resources or space were low.

"The pattern we see across many early hominins is similar to all other mammals. Speciation rates increase and then flatline, at which point extinction rates start to increase. This suggests that interspecies competition was a major evolutionary factor."

However, when van Holstein analysed our own group, Homo, the findings were "bizarre."

For the Homo lineage that led to modern humans, evolutionary patterns suggest that competition between species actually resulted in the appearance of even more new species -- a complete reversal of the trend seen in almost all other vertebrates.

"The more species of Homo there were, the higher the rate of speciation. So when those niches got filled, something drove even more species to emerge. This is almost unparalleled in evolutionary science."

The closest comparison she could find was in beetle species that live on islands, where contained ecosystems can produce unusual evolutionary trends.

"The patterns of evolution we see across species of Homo that led directly to modern humans is closer to those of island-dwelling beetles than other primates, or even any other mammal."

Recent decades have seen the discovery of several new hominin species, from Australopithecus sediba to Homo floresiensis. Van Holstein created a new database of "occurrences" in the hominin fossil record: each time an example of a species was found and dated, around 385 in total.

Fossils can be an unreliable measure of species' lifetimes. "The earliest fossil we find will not be the earliest members of a species," said van Holstein.

"How well an organism fossilises depends on geology, and on climatic conditions: whether it is hot or dry or damp. With research efforts concentrated in certain parts of the world, and we might well have missed younger or older fossils of a species as a result."

Van Holstein used data modelling to address this problem, and factor in likely numbers of each species at the beginning and end of their existence, as well as environmental factors on fossilisation, to generate new start and end dates for most known hominin species (17 in total).

She found that some species thought to have evolved through "anagenesis" -- when one slowly turns into another, but lineage doesn't split -- may have actually "budded": when a new species branches off from an existing one.*

This meant that several more hominin species than previously assumed were co-existing, and so possibly competing.

While early species of hominins, such as Paranthropus, probably evolved physiologically to expand their niche -- adapting teeth to exploit new types of food, for example -- the driver of the very different pattern in our own genus Homo may well have been technology.

"Adoption of stone tools or fire, or intensive hunting techniques, are extremely flexible behaviours. A species that can harness them can quickly carve out new niches, and doesn't have to survive vast tracts of time while evolving new body plans," said van Holstein

She argues that an ability to use technology to generalise, and rapidly go beyond ecological niches that force other species to compete for habitat and resources, may be behind the exponential increase in the number of Homo species detected by the latest study.

But it also led to Homo sapiens -- the ultimate generalisers. And competition with an extremely flexible generalist in almost every ecological niche may be what contributed to the extinction of all other Homo species.

Added van Holstein: "These results show that, although it has been conventionally ignored, competition played an important role in human evolution overall. Perhaps most interestingly, in our own genus it played a role unlike that across any other vertebrate lineage known so far."

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Climate change in the Early Holocene

New insight into how our early ancestors dealt with major shifts in climate is revealed in research, published today [27 Jan] in Nature Ecology & Evolution, by an international team, led by Professor Rick Schulting from Oxford University's School of Archaeology.

It reveals, new radiocarbon dates show the large Early Holocene cemetery of Yuzhniy Oleniy Ostrov, at Lake Onega, some 500 miles north of Moscow, previously thought to have been in use for many centuries, was, in fact, used for only one to two centuries. Moreover, this seems to be in response to a period of climate stress.

The team believes the creation of the cemetery reveals a social response to the stresses caused by regional resource depression. At a time of climate change, Lake Onega, as the second largest lake in Europe, had its own ecologically resilient microclimate. This would have attracted game, including elk, to its shores while the lake itself would have provided a productive fishery. Because of the fall in temperature, many of the region's shallower lakes could have been susceptible to the well-known phenomenon of winter fish kills, caused by depleted oxygen levels under the ice.

The creation of the cemetery at the site would have helped define group membership for what would have been previously dispersed bands of hunter-gatherers -- mitigating potential conflict over access to the lake's resources.

But when the climate improved, the team found, the cemetery largely went out of use, as the people presumably returned to a more mobile way of life and the lake became less central.

The behavioural changes -- to what could be seen as a more 'complex' social system, with abundant grave offerings -- were situation-dependent. But they suggest the presence of important decision makers and, say the team, the findings also imply that early hunting and gathering communities were highly flexible and resilient.

The results have implications for understanding the context for the emergence and dissolution of socioeconomic inequality and territoriality under conditions of socio-ecological stress.

Radiocarbon dating of the human remains and associated animal remains at the site reveals that the main use of the cemetery spanned between 100-300 years, centring on ca. 8250 to 8,000 BP. This coincides remarkably closely with the 8.2 ka dramatic cooling event, so this site could provide evidence for how these humans responded to a climate-driven environmental change.

The Holocene (the current geological epoch which began approximately 11,700 years before present) has been relatively stable in comparison to current events. But there are a number of climate fluctuations recorded in the Greenland ice cores. The best known of these is the 8,200 years ago cooling event, the largest climatic downturn in the Holocene, lasting lasted one to two centuries. But there is little evidence that the hunter-gatherers, who occupied most of Europe at this time, were much affected, and if they were, in what specific ways.

Yuzhniy Oleniy Ostrov is one of the largest Early Holocene cemeteries in northern Eurasia, with up to 400 possible graves, 177 of which were excavated in the 1930s by a team of Russian archaeologists. Based on their work, the cemetery site has an important position in European Mesolithic studies, in part because of the variation in the accompanying grave offerings. Some graves lack these entirely, to those with abundant and elaborate offerings.

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