Ancient footprints discovered in New Mexico indicate that early humans arrived in North America around 23, 000 years ago, researchers reported Thursday.

The first footprints     1     (find) in a dry lake bed in White Sands National Park in 2009. Scientists at the US Geological Survey recently analyzed seeds stuck in the footprints     2     (determine) their approximate age.     3     evidence suggested they were from 21, 130 to 22, 800 years ago.

The findings may help scientists solve a mystery that has long attracted scientists: When did people first arrive in the Americas, after     4     (leave) Africa and Asia?

Most scientists     5     (strong) believe that they came by way of a land bridge, now under water, that connected Asia to Alaska. According to various discoveries including stone tools and bones other researchers have offered a range of possible     6     (date) for human arrival in the Americas, from 13, 000 to 26, 000 years ago or more.

“    7     (we) study provides a more solid baseline for when humans were in North America, although they could have arrived even earlier,” they wrote in the journal Science, which published the study Thursday. “Fossil footprints are more     8     (rely) than cultural evidence.     9     we present here is evidence of a firm time and location.”

Based     10     the size of the footprints, researchers believe that at least some were made by children and teenagers who lived during the last ice age.

Sanxingdui is believed to have sat at the heart of the Shu state, which historians know     1     (relative) little about due to few little written records.

More than 50,000 ancient artifacts     2     (find) at Sanxingdui since the 1920s,    3     a local farmer accidentally came upon a number of relics at the site. A major breakthrough     4     (occur) in 1986, with the discovery of two ceremonial pits     5     (contain) over 1,000 items, elaborate and well-preserved bronze masks included. Many of the items are now     6     display at an on-site museum.

The site has revolutionized experts' understanding of how civilization developed in ancient China. In particular, evidence of a unique Shu culture suggested that the kingdom developed independently of neighboring societies in the Yellow River Valley, which was considered to be     7     cradle (摇篮)of Chinese civilization.

The discovery of silk fibers and the remains of textiles may also expand     8     (we) understanding of the Shu. Head of the excavation team, Tang Fei, said in a press conference that the discovery indicates that the kingdom “was one of the important     9    (origin) of silk in ancient China”.

Though not yet     10    (recognize) as a UNESCO World Heritage Site, Sanxingdui is on the organization's "tentative list"   for possible future inclusion.