So, what does make us human? Why are humans so different from nonhuman primates with regards to phenotype, intelligence, and behavior? This article examines how the evolution in a particular region of the primate brain, the neocortex, may account for differences in the cognitive abilities between humans and our closest ancestor, the chimpanzee.

The neocortex is an integral part of the mammalian brain, and was the most recent part of the cerebral cortex to evolve. It can be divided into four lobes—frontal, parietal, occipital, and temporal lobes—which perform a variety of functions ranging from visual acuity to conscious thought [3]. The neocortex is initially thought to have emerged as a simple six-layered structure made up of mature neurons, but it since has evolved into more complex structures [4]. The neocortex ratio—the ratio of the size of the neocortex to the brain volume—has also progressively increased from macaque monkeys to human. Scientists believe that these two disparities in primates’ brains account for the substantial differences in intelligence between humans and nonhuman primates.

While the size of the neocortex is significantly greater in humans than in chimpanzees, there exist several attributes that have retained their similarities throughout primate evolution. A common theme in all primates is the way neurons emerge and position themselves in the neocortex. All neurons began to develop in the embryonic ventricular (VZ) and subventricular (SVZ) zones. The progenitors that appear in these zones develop into the projection neurons that eventually migrate to their respective neocortical layers VI to II in an “inside-out” fashion [5].

The enlargement of the cortical surface of the neocortex can be explained primarily by the “radial unit hypothesis,” which states that the neural progenitors symmetrically divide into radial cortical columns. The radial glial cells further divide asymmetrically to produce intermediate progenitor cells, which then divide symmetrically to form neural projection cells. An increase in the number radial glial cells that rise from progenitors is what accounts for the increase in the surface area of the cortical surface without a simultaneous increase in the thickness. During primate evolution, these cortical surfaces expanded into distinct arrangements. A larger size of cortical surface allows for more neural connections, improving cognitive abilities [6, 7].

The use of modern bioinformatics tools, such as RNA sequencing and microarray analysis, provide insights into neocortical development and is particularly significant in elucidating how the modern human brain emerged from primates, especially chimpanzees. The data can then be analyzed.

According to a 1975 paper by King and Wilson, nearly all of the protein-coding sequences in humans and chimpanzees are the same. About 98% of our DNA is shared with that of the chimpanzee. However, these similar genetic sequences cannot and do not account for our drastic phenotypic differences. King and Wilson further clarified that the regulation of gene expression plays a far greater role in accounting for those phenotypic differences. Since changes in gene expression most prominently affect phenotype during the early stages of neocortical development, the importance of examining the embryonic stages of neurogenesis should be emphasized [8].

Preparing RNA sequencing libraries is one of the many methods to determine which part of the genome is expressed, since examining the sequences of RNA gives us a snapshot of the cell at any given moment in time. Often, making RNA-seq libraries involves converting raw RNA into DNA, which is a more stable macromolecule. Adaptors are then tagged onto the cDNA for bridge amplification. By measuring the light intensity and emitted wavelength of fluorescently tagged nucleotides, the DNA can be sequenced [10].

After sequencing the DNA, the sequences need to be mapped to a reference genome. Differential gene expression analysis is primarily interested in determining the relative DNA differences in expression levels between reads. However, inherent bias in the data needs to be corrected. First, longer reads will tend to have more transcripts (the longer the read, the higher probability that a transcript will match to the read on the reference genome). Furthermore, the library preparation with the higher concentration will have more number of transcripts. Count normalization, which performs various statistical algorithms, can account for this inherent bias, giving a more accurate measure of the relative differences in the gene expression of the reads [11].

To determine whether a difference in the expression levels between genes exists, a statistical test, typically a p test, is performed. By comparing the p values, for example, for the different library preparations of cells over various stages of potency, the down regulation and up regulation of certain genes can be determined [12]. So far, research indicates that the differences in the neocortex of primates stem from differential gene expression in the stem cells of the VZ. The information encoded in the regulatory genes is carried to the cortical plate by radial glial cells [13].

Ultimately, the differences in cognitive abilities and social behaviors between humans and chimpanzees can be explained by two major factors at this point. First, larger brain development during the early stages of embryonic growth in humans results in increased cerebral volume, but in nonhuman primates, brain development occurs to a much lesser extent. Second, images of the human brain display a greater number of neural connective fibers than images of nonhuman primates, which may account for our higher level of cognitive ability.

By further examining the timing and levels of expression in a variety of genes and by analyzing brain images during early embryonic development, scientists hope to find the answer to the question that remains partially unanswered: what makes humans unique?