Abstract designs scratched on mineral pigment show up in Africa about 75,000 years ago and are widely accepted by archaeologists as evidence for symbolism and language. "From this point onward there is a growing variety of new types of artifacts that indicates a thoroughly modern capacity for novelty and invention."
While crude stone tools crafted by human ancestors beginning about 2.5 million years ago likely were an indirect consequence of bipedalism -- which freed up the hands for new functions -- the first inklings of a developing super-brain likely began about 1.6 million years ago when early humans began crafting stone hand axes, thought by Hoffecker and others to be one of the first external representations of internal thought.
D - the evidence in language itself is indirect and tenuous.
Scholars estimate that PIE may have been spoken as a single language (before divergence began) around 3700 BC, though estimates by different authorities can vary by more than a millennium.
Proposed genetic connections
Many higher-level relationships between Proto-Indo-European and other language families have been proposed, but these hypothesized connections are highly controversial. A proposal often considered to be the most plausible of these is that of an Indo-Uralic family, encompassing PIE and Uralic. The evidence usually cited in favor of this consists in a number of striking morphological and lexical resemblances. Opponents attribute the lexical resemblances to borrowing from Indo-European into Uralic. Frederik Kortlandt, while advocating a connection, concedes that "the gap between Uralic and Indo-European is huge", while Lyle Campbell, an authority on Uralic, denies any relationship exists.
Other proposals, further back in time (and proportionately less accepted), link Indo-European and Uralic with Altaic and the other language families of northern Eurasia, namely Yukaghir, Korean, Japanese, Chukotko-Kamchatkan, Nivkh, Ainu, and Eskimo-Aleut, but excluding Yeniseian (the most comprehensive such proposal is Joseph Greenberg's Eurasiatic), or link Indo-European, Uralic, and Altaic to Afro-Asiatic and Dravidian (the traditional form of the Nostratic hypothesis), and ultimately to a single Proto-Human family.
A more rarely mentioned proposal associates Indo-European with the Northwest Caucasian languages in a family called Proto-Pontic.
Indo-Uralic is a hypothetical language family consisting of Indo-European and Uralic.
A genetic relationship between Indo-European and Uralic was first proposed by the Danish linguist Vilhelm Thomsen in 1869 (Pedersen 1931:336) but was received with little enthusiasm. Since then, the predominant opinion in the linguistic community has remained that the evidence for such a relationship is insufficient. However, a minority of linguists has always taken the contrary view (e.g. Henry Sweet, Holger Pedersen, Björn Collinder, Warren Cowgill and Jochem Schindler).
History of opposition to the Indo-Uralic hypothesis
The history of early opposition to the Indo-Uralic hypothesis does not appear to have been written. It is clear from the statements of supporters such as Sweet that they were facing considerable opposition and that the general climate of opinion was against them, except perhaps in Scandinavia.
Károly Rédei, editor of the standard etymological dictionary of the Uralic languages (1986a), rejected the idea of a genetic relationship between Uralic and Indo-European, arguing that the lexical items shared by Uralic and Indo-European were due to borrowing from Indo-European into Proto-Uralic (1986b).
Perhaps the best-known critique of recent times is that of Jorma Koivulehto, issued in a series of carefully formulated articles. Koivulehto’s central contention, agreeing with Rédei's views, is that all of the lexical items claimed to be Indo-Uralic can be explained as loans from Indo-European into Uralic (see below for examples).
D - looks like the jury might be out on this forever.