Could you please point out those similarities? My knowledge on South American tribes is pretty sketchy, but what you've said sounds very very interesting.
I was thinking of North and Central American tribes moreso than South, though that might also make for interesting comparison. I'm afraid I don't have my notes on this handy, so I will need to look up some things to get the details accurate, but off the top of my head for the sake of discussion, it seems like the similarities I noticed were in things like dress/fashion, warfare customs (incl. dress/fashion related to warfare customs like warpaint--I believe warpaint was one of the first things I noticed that got me thinking about this subject), burial customs, and certain religious elements. I'm trying to find a good link right now that sums up some of this stuff. This link isn't precisely on that subject, but it includes some relevant information:
http://www.tattooheaven.com/CentAsia.html
Ancient Tatooed People of Central Asia
TATTOOED MUMMIES OF CENTRAL ASIA
Mummies have been found in Central Asia! Though these curious Caucasian mummies in the deserts of western China were first discovered by Western archaeologists in the early 20th century, they were considered anomalies---perhaps just ancient travelers or immigrants. Over the past thirty years Chinese archeologists have unearthed hundreds more of these mummified Caucasoids (as well as abundant skeletal remains amounting to thousands of ancient individuals) in the Tarim basin of Central Asia. The Tarim is in the huge Taklamakan desert in the western Chinese province of Xinjiang, formerly known as eastern Turkestan.
Today the ancient Chinese texts which speak of legendary tall people with red hair and green eyes (formerly denigrated as mere "myths") are being reinterpreted. They are not just imaginary tales as has been assumed until recently, but they tell of the very real Tocharian-branch Indo-European people, relatives of the Celts and Scythians, who possibly controlled the Silk Road during Middle and Egyptian New Kingdom times, and down to the Classical Greek era. They certainly would have been involved in the transmission of technology and culture between East and West at a very early date.
The time span of the Central Asian Caucasoids is from 2500 BC to 400 B.C. The location is within a few hundred miles of the Altai "Scythian" burials which date from approximately 500-300 BC. There is definitly some connection here. There is also a connection between the Taklamakan people and the Crimean Scyths, the Celts and the Picts. They likely influenced the "indigenous" tattooing of the tribal peoples of India, and possibly are antecedent to the Jomon culture of Japan (ancestors of the tattooed Ainu). There is credible evidence that some of the tattooing tribes of northern Asia migrated eastward to become certain tribes in the Americas as well.
[SNIP]
Compare that with this:
http://www.tylwythteg.com/pict1.html
But in interpreting these comments, it must be understood that the classical "anthropological" tradition involved a great deal of repeating and interpreting the claims of earlier writers, and extremely little direct observation and eye-witness report. An example of the pseudo-history repeated by Bede claims a Scythian origin for the Picts, but this seems no more than an attempt to connect them with another people described in classical writings as "Picti". Other pseudo-histories carefully list wanderings and emigrations of "the Picts" that would connect them with every place or ethnic name resembling "pict" (such as the Pictones of Gaul, whose name became modern Poictou) and every mention of skin-painting or tattooing. A great deal of the material repeated by Isidore and Bede and similar writers is demonstrably false. Other parts can be corroborated by archaeological methods. But any use of this sort of material must involve several large grains of salt. Of all the early writers that mention "painting", only Caesar seems to have been an eye-witness, and his observations would have been concerned with the inhabitants of southern Britain, the Celtic peoples that he explicitly calls "Britanni".
That's just one example I happen to have handy. I will need to collect my notes on some others. Here's one other I happen to have at hand which is on a slightly different aspect of this. I was relating this to some of the discussion of ancient American runes in Barry Fell's work:
http://www.crystalinks.com/runes.html
[SNIP]
When Bisop Wulfila made his translation of the Bible into fourth century Gothic, he rendered St. Mark's "the mystery of the kingdom of God" using "runa" for "mystery.
Eight centuries earlier, when Greek historian Herodotus traveled around the Black Sea, he encountered descendants of Scythian tribesmen who crawled under blankets, smoked themselves into a stupor, and cast marked sticks in the air and "read" them when they fell. These sticks were used as Rune sticks.
There is no firm agreeement among scholars as to where and when runic writing first made its appearance in Western Europe. Before Germanic peoples possessed any form of script, they used pictorial symbols that they scratched onto rocks.
[SNIP]
By 100 AD the Runes were already becoming widely known on the European Continent. They were carried from place to place by traders, adventurers, and warriors, and eventually by Anglo-Saxon missionaries.
[SNIP]
The Curse Of The Red-Headed Mummy
I notice in the article you posted a reference to the Picts being related to the Scythians. I actually believe they are closer related to the Xiongnu
Oetzi the Iceman also has tattoos...they are made at well known acupuncture points.