They are found in different environments ranging from acidophilic thermophiles to mesophiles and psychrophiles and with different types of metabolism, predominantly anaerobic and chemosynthetic.

[4] TACK is a clade that is sister to the Asgard branch that gave rise to the eukaryotes.

It has been proposed that the TACK clade be classified as Crenarchaeota and that the traditional "Crenarchaeota" (Thermoproteota) be classified as a class called "Sulfolobia", along with the other phyla with class rank or order.

[6] The relationships are roughly as follows: "Korarchaeota" "Bathyarchaeota" "Aigarchaeota" Nitrososphaerota "Verstraetearchaeota" Thermoproteota "Geoarchaeota" "Marsarchaeota" Conexivisphaerales Nitrososphaerales Nitrosopumilales Thermoproteales Fervidicoccales Desulfurococcales 2 Desulfurococcales Sulfolobales "Panguiarchaeales" "Korarchaeales" "Bifangarchaeales" [B24] "Hecatellales" [B25] "Xuanwuarculales" [RBG-16-48-13] "Houtuarculales" [40CM-2-53-6] "Wuzhiqiibiales" [TCS64] "Zhuquarculales" [EX4484-135] "Bathyarchaeales" [B26-1] "Caldarchaeales" "Geothermarchaeales" Conexivisphaerales Nitrososphaerales "Nezhaarchaeales" "Culexarchaeles" "Methanomethylicales" "Gearchaeales" Thermofilales Thermoproteales "Marsarchaeales" Sulfolobales The eocyte hypothesis proposed in the 1980s by James Lake suggests that eukaryotes emerged within the prokaryotic eocytes.

[15] One piece of evidence supporting a close relationship between TACK and eukaryotes is the presence of a homolog of the RNA polymerase subunit Rbp-8 in Thermoproteota but not in Euryarchaea.