It recently culminated as the nitrogen-included ultrananocrystalline diamond (N-UNCD), which was reported to be the only successful form of diamond for neural stimulation and recording. [14] The N-UNCD is fabricated through the intriguing bottom-up evolution of the diamond-graphite hybrid nanostructure under chemical vapor deposition (CVD). [4][5][6][7][8][9] While the material is indeed outstanding in the neural stimulation and recording, there's still an important issue to be resolved: their performance for the simultaneous electrochemical detection of the neural transmitter, for example, dopamine (DA) in presence of ascorbic acid (AA) and uric acid (UA) is still far from satisfactory, [26,27] as compared to that of the nanocarbon-powder-based porous thin films. [28] Furthermore, the detection of AA in presence of interference from DA and UA, which is inevitable in many biological samples, [29] is particularly challenging, relative to those of DA and UA, which might be attributed to the lack of π-π molecular bonding route of AA absorption on carbon electrode. [30,31] Such challenging situation was persistent until recently despite the decade-long presence of the diamond-graphite hybrid nanostructured thin film, and one might attribute it to the lack of the fundamental understanding of the bottom-up evolution mechanism of the hybrid film, which is essential for a proper optimization of materials performance throughThe diamond-graphite hybrid thin film with low-dimensional nanostructure (e.g., nitrogen-included ultrananocrystalline diamond (N-UNCD) or the alike), has been employed in many impactful breakthrough applications. However, the detailed picture behind the bottom-up evolution of such intriguing carbon nanostructure is far from clarified yet. Here, the authors clarify it, through the concerted efforts of microscopic, physical, and electrochemical analyses for a series of samples synthesized by hotfilament chemical vapor deposition using methane-hydrogen precursor gas, based on the hydrogen-dependent surface reconstruction of nanodiamond and on the substrate-temperature-dependent variation of the growth species (atomic hydrogen and methyl radical) concentration near substrate. The clarified picture provides insights for a drastic enhancement in the electrochemical activities of the hybrid thin film, concerning the detection of important biomolecule, that is, ascorbic acid, uric acid, and dopamine: their limits of detections are 490, 35, and 25 nm, respectively, which are among the best of the all-carbon thin film electrodes in the literature. This work also enables a simple and effective way of strongly enhancing AA detection.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202105087.