Being terrestrial is one obvious requirement. No fully aquatic species could master fire and thus develop metallurgy.
Do the authors of the study think this is intelligently designed? They almost say so.
The importance of metals, particularly iron, and the importance of the discovery of metallurgy can hardly be exaggerated.
You may have noticed a strange loop which arises spontaneously in many sectors of the intelligent design debate.
The five authors, led by Thomas Y. Lo, cover the range of evidence for intelligent design in under 150 pages.