Sunday, 20 August 2017

Retracing the First Americans' Footsteps

Postglacial viability and colonization in North America’s ice-free corridor | Nature

One of the enduring mysteries of the story of the human migration into North America is the exact route taken and the timing of the migration(s). There is now no real doubt that North America was populated first with a subsequent rapid spread into Central and South America, and that the people came from Siberia via the Beringia; the land bridge between Siberia and Alaska, the remnants of which are the Aleutian Islands.

Looking south through what was once the “ice-free corridor” in present-day Canada. A new study suggests that humans couldn’t have traversed through the corridor until about 12,600 years ago - too late to have been the route taken by the first inhabitants of North America.

Credit: Mikkel Winther Pedersen

Now the relatively new techniques for recovering and analysing environmental DNA (eDNA) is helping to fill in these gaps.

eDNA is the DNA from decayed plant and animal matter which remains in the soil, preserved by particles adhering to it's electrostatically charged surface. Recovering and analysing this eDNA 'fossil' evidence is now being used to analyse cave floor debris, for example. Although the DNA of individuals is probably not recoverable, what can be recovered is enough to identify the species, and, in the case of mtDNA, is helping to identify particular populations and relationships with nearby populations.

Exposed by lowered sea-level due to so much water being land-locked in glaciers, Beringia was inhabited at the time of the last glacial maximum when, paradoxically, because of ocean currents and aridity it was relatively ice-free with a mild climate. But, with the Canadian glaciers at their maximum, they were an effective barrier to further migration by the fauna that entered from Siberia and became isolated in Beringia.

And timing is crucial because the known archaeology from North America needs to marry up with the evidence for climate change and ice-free passages through the glaciers, or via coastal routes. If people were present in North America before any such passage opened up, then they can't have got there via that route, even if others did later. The alternative is that they came down the coast.

This paper 2016 attempts to reconcile conflicting evidence by examining pollen, etc from the sediment in the beds of an ancient lake, together for the first time with environmental DNA (eDNA).

Before 12,600 years ago, the researchers say, the more interior route into the Americas was blocked. Instead, early settlers may have taken a coastal route as early as 14,700 years ago.

Credit: Mikkel Winther Pedersen
During the Last Glacial Maximum, continental ice sheets isolated Beringia (northeast Siberia and northwest North America) from unglaciated North America. By around 15 to 14 thousand calibrated radiocarbon years before present (cal. kyr BP), glacial retreat opened an approximately 1,500-km-long corridor between the ice sheets. It remains unclear when plants and animals colonized this corridor and it became biologically viable for human migration. We obtained radiocarbon dates, pollen, macrofossils and metagenomic DNA from lake sediment cores in a bottleneck portion of the corridor. We find evidence of steppe vegetation, bison and mammoth by approximately 12.6 cal. kyr BP, followed by open forest, with evidence of moose and elk at about 11.5 cal. kyr BP, and boreal forest approximately 10 cal. kyr BP. Our findings reveal that the first Americans, whether Clovis or earlier groups in unglaciated North America before 12.6 cal. kyr BP, are unlikely to have travelled by this route into the Americas. However, later groups may have used this north–south passageway.

It had been thought until recently the the 'Clovis people', big-game hunters who were present in the Southern United States by about 13,000 years ago, however, recent finds have shown that humans were present at least 15,000 years ago. So, the key question for deciding which route was the most likely one is when exactly did the ice-free corridor become available and suitable for a 1500 kilometre (930 mile) migration. It has been estimates that a long journey such as that would have required the existence of big game to sustain the travellers.

To build up a time-line of the corridor's past, the team examined nine sediment cores from two lakes near the narrowest point which would have been the last point to have opened up; the final gateway to the south. Taken together, this pollen, fossil and eDNA evidence builds up the following time line:

  • For about 700 years after the ice melted there was little by the way of vegetation or animal life.
  • By about 12,600 years ago, steppe plants such as sagebrush appear, followed soon by animals such as woolly mammoth, bison and jackrabbits.
  • 12,400 years ago the plants are dominated by aspen and poplar trees, in other words, the corridor has become a broad-leaf woodland. Soon after, elk and moose arrive.
  • By 11,600 years ago the treas had been replaced by spruce and pine conifers transforming it from a broad-leaf to a coniferous woodland.

But this opening up and establishment of an ecosystem capable of sustaining humans on this journey comes too late! There is no doubt that humans were established far to the south well before these changes occur. So, the most likely route, on this finding is the coastal one, bypassing the glacial barrier.

However, there is still some hope for the corridor route because of tentative evidence that it opened up slightly earlier. Comparison of the mtDNA of northern and southern bison suggests they migrated through by about 13,000 years ago; there is a fragment of poplar tree dated at 13,700 years old and a 13,100 year-old bison found near the bottleneck.

Of course, for a creationist, the only explanation now needed is why all this evidence is there from 11-14,000 years ago when Earth hadn't yet been created and a subsequent global flood should have destroyed it all. I dare say Ham, Hovind, Comfort and the professional liars at the Discovery Institute can offer and explanation...

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