* Astronomy

The discovery by Andrew Solow of Woods Hole Oceanographic Institute, US, David Roberts of the Royal Botanic Garden, Kew and Karen Robbirt of the University of East Anglia (UEA) is published this week in Proceedings of the National Academy of Sciences (PNAS).

The accepted view had previously been that the wild horses became extinct long before the extinction of mammoths and the arrival of humans from Asia - ruling out the possibility that they were over-hunted by man. One theory had been that a period of climate cooling wiped them out.
However, the researchers have discovered that uncertainties in dating fossil remains and the incompleteness of fossil records mean that the survival of the horse beyond the arrival of humans cannot be ruled out.
The PNAS paper develops a new statistical method to help resolve the inherent problems associated with dating fossils from the Pleistocene period. The aim is to provide a far more accurate timetable for the extinction of caballoid horses and mammoths and, ultimately, the cause.

"This research is exciting because it throws open the debate as to whether climate change or over-hunting may have led to the extinction of pre-historic horses in North America" - UEA's Karen Robbirt.

The Pleistocene period refers to the first epoch of the Quarternary period between 1.64 million and 10,000 years ago. It was characterised by extensive glaciation of the northern hemisphere and the evolution of modern man around 100,000 years ago.
It is known that the end of the Pleistocene period was a time of large-scale extinctions of animals and plants in North America and elsewhere but the factors responsible have remained open to question, with climate change and over-hunting by humans the prime suspects.

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Generations of schoolbooks have portrayed the arrival of the first modern humans to America as an epic ice-age hike across a land bridge from Siberia to Alaska, then a dash between glaciers covering the west and east of Canada.
But scientists who have devoted much of their careers over the past several decades to better understanding of the peopling of the Americas are increasingly doubtful that the first arrivals only walked into the hemisphere, if they walked at all. Instead, evidence is growing that they paddled, or floated, much of the way, perhaps via the Atlantic as well as the Pacific.

"The coastal-migration theory has yet to be proven with hard evidence, but we have been finding earlier and more widespread evidence for coastal settlement around the Pacific Rim" - Jon Erlandson, anthropologist at the University of Oregon who spoke during the annual meeting of the American Association for the Advancement of Science at the weekend.

His team shows how migration routes may have followed giant kelp forests growing along Pacific Rim coastlines even in the deepest freeze of the last ice age more than 20,000 years ago.

Paleoanthropology has traditionally been mostly about stone tools, particularly blades more or less skilfully flaked into knives, spear points and axes. How a tool was made, and from what sort of material, tells experts a lot about who made it; a little radiocarbon dating of organic material, often charcoal, found around the tools, tells them when they were made, usually within a few hundred years each way.

Until recently, most of the older stone implements found in North and Central America seemed to have been made with the same technique by people dubbed the Clovis culture, for the first material found in New Mexico during the 1930s, and dated back to no more than about 11,500 years ago.
A few sites in Virginia, Pennsylvania and elsewhere that held tools of different styles that seemed to be as much as 10,000 years older had been largely written off, until more old tools coming from layers dating back 14,000, 17,000, even 50,000 years ago started turning up along the East Coast. And diverse sites in South America have yielded artefacts dating to 33,000 years ago, although controversies about methods used to date sites are a staple of the field.
Dennis Stanford of the Smithsonian Institution's National Museum of Natural History in Washington, who has spent much of his career studying cultures around the Arctic, was among the first to note that spear points and other tools found on the East Coast have a lot in common with points made by a Stone Age culture known as Solutrean, centred in what's now South-western France.

In recent years, he and colleagues have found strong evidence in the form of bones, paintings and other items in coastal caves of Spain that Solutreans used harpoons and boats to go out into icy waters and hunt seals, walrus and auks. And they've worked with ancient-climate specialists to establish that sea icepack extended that far south in the Atlantic during the last ice age.
One of the problems in proving this frozen-highway theory in the Atlantic has been that any coastal camps the hunters may have used now lie submerged well out on the continental shelf due to rising sea level. However, Stanford noted that some promising artefacts - along with walrus bones - have turned up recently at new sites around the Chesapeake Bay, for instance.
In the Pacific, Erlandson and many other paleoanthropologists have found and excavated camps of marine hunters at many spots along the coast, particularly offshore islands. What's been missing is something to connect them as a migration route.
It is known that seafaring people lived at least as far north as Japan at the height of the last glacial period, but a team of marine biologists and other specialists helped Erlandson demonstrate how the fish- and mammal-rich kelp forests ran in an arch all the way from the Kurile Islands to Alaska and along all or most of the Pacific Coast. The kelp beds not only ensured food, but also could have helped protect small boats from big waves and served as mooring points.

"The fact that these productive kelp forests are found adjacent to some of the earliest coastal archaeological sites in the Americas really enhances the argument that the first Americans didn't walk here, they floated. In essence, they may have utilised a sort of kelp highway" - Jon Erlandson.

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Thick-skinned bottle gourds widely used as containers by prehistoric peoples were likely brought to the Americas some 10,000 years ago by individuals who arrived from Asia, according to a new genetic comparison of modern bottle gourds with gourds found at archaeological sites in the Western Hemisphere. The finding solves a longstanding archaeological enigma by explaining how a domesticated variant of a species native to Africa ended up millennia ago in places as far removed as modern-day Florida, Kentucky, Mexico and Peru.

The work, by a team of anthropologists and biologists from Harvard University, the Smithsonian Institution's National Museum of Natural History, Massey University in New Zealand and the University of Maine, appears this week on the web site of the Proceedings of the National Academy of Sciences.

Integrating genetics and archaeology, the researchers assembled a collection of ancient remnants of bottle gourds from across the Americas. They then identified key genetic markers from the DNA of both the ancient gourds and their modern counterparts in Asia and Africa before comparing the plants' genetic make-up to determine the origins of the New World gourds.

"For 150 years, the dominant theory has been that bottle gourds, which are quite buoyant and have no known wild progenitors in the Americas, floated across the Atlantic Ocean from Africa and were picked up and used as containers by people here. Much to our surprise, we found that in every case the gourds found in the Americas were a genetic match with modern gourds found in Asia, not Africa. This suggests quite strongly that the gourds that were used as containers in the Americas for thousands of years before the advent of pottery were brought over from Asia" - Noreen Tuross, the Landon T. Clay Professor of Scientific Archaeology in Harvard's Faculty of Arts and Sciences.

The researchers say it's possible the domesticated gourds -- differentiated from wild bottle gourds by a much thicker rind -- were conveyed to North America by people who arrived from Asia in boats or who walked across an ancient land bridge between the continents, or that the gourds floated across the Bering Strait after being transported by humans from their native Africa to far northeastern Asia.

"This finding paints a new picture of the founding of the Americas. These people did not arrive here empty-handed; they brought a domesticated plant and dogs with them. They arrived with important tools necessary to survive and thrive on a new continent, including some knowledge of and experience with plant domestication" - Bruce Smith, co-author , Smithsonian Institution.

Thought to have originated in Africa, bottle gourds (Lagenaria sicereria) have been grown worldwide for thousands of years. The gourds have little food value but their strong, hard-shelled fruits were long prized as containers, musical instruments and fishing floats. This lightweight "container crop" would have been particularly useful to human societies before the advent of pottery and settled village life, and was apparently domesticated thousands of years before any plant was domesticated for food purposes.

Radiocarbon dating indicates that bottle gourds were present in the Americas by 10,000 years ago and widespread by 8,000 years ago. Some of the specimens studied by the team were not only the oldest bottle gourds ever found but also quite possibly the oldest plant DNA ever analysed. The newest of their archaeological samples, a specimen found in Kentucky, was just 1,000 years old -- suggesting the gourds were used in the New World as containers for at least 9,000 years.

Alleged footprints of early Americans found in volcanic rock in Mexico are either extremely old - more than 1 million years older than other evidence of human presence in the Western Hemisphere - or not footprints at all, according to a new analysis published this week in Nature.

The study was conducted by geologists at the Berkeley Geochronology Centre and the University of California, Berkeley, as part of an investigative team of geologists and anthropologists from the United States and Mexico.
Earlier this year, researchers in England touted these "footprints" as definitive proof that humans were in the Americas much earlier than 11,000 years ago, which is the accepted date for the arrival of humans across a northern land-bridge from Asia.

These scientists, led by geologist Silvia Gonzalez of Liverpool's John Moores University, dated the volcanic rock at 40,000 years old. They hypothesized that early hunters walked across ash freshly deposited near a lake by volcanoes that are still active in the area around Puebla, Mexico. The so-called footprints, subsequently covered by more ash and inundated by lake waters, eventually turned to rock.
But Paul Renne, director of the Berkeley Geochronology Centre and an adjunct professor of earth and planetary science at UC Berkeley, and his colleagues in Mexico and at Texas A&M University report in the Dec. 1 issue of Nature a new age for the rock: about 1.3 million years.

"You're really only left with two possibilities. One is that they are really old hominids - shockingly old - or they're not footprints." - Paul Renne.

Renne's colleagues are Michael R. Waters, director of the Centre for the Study of the First Americans at Texas A&M University; Joaquin Arroyo-Cabrales and Mario Perez-Campa of the Mexican National Institute of Anthropology and History; Patricia Ochoa Castillo of the Mexican National Museum of Anthropology; and UC Berkeley graduate students Joshua M. Feinberg and Kim B. Knight. The Berkeley Geochronology Centre, located a block from the UC Berkeley campus, is one of the world's pre-eminent anthropological dating laboratories.
Paleoanthropologist Tim White, professor of integrative biology at UC Berkeley, is familiar with the "so-called footprints" and knows Renne well, frequently collaborating with him in the dating of million-year-old sediments in an area of Ethiopia where White has excavated numerous fossils of human ancestors. He is not surprised at the new finding.

"The evidence (the British team) has provided in their arguments that these are footprints is not sufficient to convince me they are footprints. The evidence Paul has produced by dating basically means that this argument is over, unless indisputable footprints can be found sealed within the ash" - Tim White.

Renne determined the new date using the argon/argon dating technique, which reliably dates rock as young as 2,000 years or as old as 4 billion years. The British-led researchers, however, relied mainly on carbon-14 dates of overlying sediments. Carbon-14 cannot reliably date materials older than about 50,000 years.
The idea for another test that, it turns out, throws more cold water on the footprint hypothesis came to Renne one morning in the shower. Many rocks retain evidence of their orientation at the moment they cool in the form of iron oxide grains magnetized in a direction parallel to the Earth's magnetic field at the time of cooling. Because the Earth's field has repeatedly flipped throughout the planet's history, it is possible to date rock based on its magnetic polarity.
Feinberg found that the rock grains in the volcanic ash had polarity opposite to the Earth's polarity today. Since the last magnetic pole reversal was 790,000 years ago, the rock must be at least that age. Because the Earth's magnetic polarity changes, on average, every 250,000 years, the argon/argon date is consistent with a time between 1.07 and 1.77 million years ago when the Earth's polarity was opposite to that of today.
Moreover, Feinberg found that each individual grain in the rock is magnetized in the same direction, meaning that the rock has not been broken up and reformed since it was deposited. This makes extremely unlikely the possibility that the original ash had been weathered into sand that early humans walked through before the sand was welded into rock again.

"Imagine two-millimetre-wide BBs cemented together where they're touching. The paleomagnetic data tell us that these things did not move around at all since they were deposited. They haven't been eroded and redeposited anywhere else. They fell while they were still hot, which raises the question of the validity of the footprints. If they were hot, why would anybody be walking on them?" - Joshua M. Feinberg.

The British researchers, funded by the United Kingdom's Natural Environment Research Council, have promoted their hypothesis widely, most prominently at a July 4, 2005, presentation and press conference at the Royal Society's Summer Science Exhibition 2005 in London. The team, which includes Gonzalez as well as Professor David Huddart from John Moores University, also involves scientists from Bournemouth University, the University of Oxford and the Australian National University. They have yet to publish a peer-reviewed analysis of the footprints.
In all, the British team claims to have found 250 footprints - mostly human, but also dog, cat and cloven-hoofed animal prints - in a layer of volcanic ash deposited in a former lake bed now exposed near a reservoir outside Puebla. Its dating techniques returned a date of 40,000 years ago, in contrast to the oldest accepted human fossil from the Americas, an 11,500-year-old skull.

This makes the rock "one of the most important areas in the study of early human occupation in the Americas and would support a much earlier human migration than is currently accepted".

"Accounting for the origin of these footprints would require a complete rethink on the timing, route and origin of the first colonization of the Americas" - Matthew Bennett, Bournemouth.

Renne, Knight, Waters and the Mexico City archaeologists visited the site at the Toluquilla quarry last year while collecting rocks from another anthropological site across the reservoir. Renne noted that the black, basaltic rock is very tough and is mined in slabs for building. Pre-Columbian Mexicans also constructed buildings from the rock, which they called xalnene, meaning "fine sand" in the Nahuatl language. Today, trucks headed toward the quarry routinely drive across the xalnene tuff in which the alleged footprints are found, and the rock itself is pockmarked with many depressions in addition to the alleged footprints.

"They're scattered all over, with no more than two or three in a straight line," which would be expected if someone had walked through the ash, Renne said. If the depressions were footprints, modern humans could not have made them, he noted, since even in Africa, Homo sapiens did not appear until about 160,000 years ago. Given the age of the volcanic rock and lacking other evidence of early human ancestors in the Americas 1.3 million years ago, the researchers wrote in their paper, "we consider such a possibility to be extremely remote."

Many palaeontologists have withheld judgment on the alleged footprints, awaiting good geological dates.

"With this study, we're trying to nip any misrepresentation in the bud" - Joshua M. Feinberg.