New research provides the strongest evidence yet that an ancient human ancestor, *Sahelanthropus tchadensis*, walked upright on two legs between 6.7 and 7.2 million years ago^[1][2]. This discovery, based on a detailed analysis of fossils found in Chad's Djurab desert, pushes back the timeline for habitual bipedalism to near the very root of the human family tree^[3][2]. Scientists say this ape-like early hominin possessed a chimpanzee-sized brain but had key anatomical features adapted for terrestrial bipedalism^[1][4][2]. ### Breakthrough in Bipedalism Research Anthropologists from New York University led the recent study^[1][5]. They focused on a femur (thigh bone) and two partial forearm bones of *Sahelanthropus tchadensis*^[1][5]. These bones were recovered from the Toros-Menalla site in Chad in 2001^[1][6]. For over two decades, experts debated whether this early hominin truly walked upright^[1][6]. The new analysis used advanced 3D shape modeling and anatomical trait comparisons^[1][3]. Researchers found a small bony structure on the femur, called a femoral tubercle^[1][3]. This tubercle serves as the attachment point for the iliofemoral ligament, the strongest ligament in the human body^[3][5]. Scientists have only identified this ligament in bipedal hominins, and it is critical for standing and walking upright^[5][7]. Dr. Scott Williams, an associate professor at New York University and lead author, explained the significance^[5][2]. "Our analysis of these fossils offers direct evident that *Sahelanthropus tchadensis* could walk on two legs," Williams said^[1][3]. He added that this demonstrates bipedalism evolved early in our lineage^[1][8]. The findings reinforced models placing chimpanzee-like creatures near the root of the human family tree^[1]. The study, published in *Science Advances*, revealed other hallmarks of upright walking^[5][7]. These include a natural twist in the thigh bone that helps legs point forward^[5][7]. Strong gluteal muscles also aided hip stability, standing, walking, and running^[5][7]. Despite its ape-like appearance and small brain, *Sahelanthropus tchadensis* was adapted for bipedal posture and movement on the ground^[1][5][4]. However, *Sahelanthropus tchadensis* likely engaged in a hybrid lifestyle^[4]. It still spent significant time in trees for foraging and safety^[1][8]. Its limb proportions, with a relatively long femur compared to its ulna (forearm bone), suggest an early shift towards bipedal movement^[6][8]. This indicates upright walking was part of its behavior, even if not exclusive^[4][8][7]. ### Other Early Upright Walkers Before the strong evidence for *Sahelanthropus tchadensis*, other species were considered among the earliest bipeds^[9][10]. *Orrorin tugenensis*, found in Kenya and dated to about 6 million years ago, also shows evidence of bipedalism^[9][11][12]. Fossils include partial femora (thigh bones) that suggest bipedal locomotion, though some researchers remain skeptical^[9]. The *Orrorin tugenensis* femur differs from those of apes and modern humans^[11]. It strongly resembles *Australopithecus* and *Paranthropus*^[11]. This indicates *Orrorin tugenensis* was bipedal but not more closely related to *Homo* than to *Australopithecus*^[11]. Like *Sahelanthropus*, *Orrorin tugenensis* likely climbed trees as well^[12][13]. Its femoral neck is elongated, similar in shape and length to modern humans and Australopithecines^[14]. Another important species is *Ardipithecus ramidus*, which lived around 4.4 million years ago in Ethiopia^[15][10][16]. The partial skeleton nicknamed "Ardi" shows a mix of features for both tree-climbing and bipedal activity^[16][17]. While *Ardipithecus* had an offset grasping big toe useful for climbing, analysis shows it also used its big toe to help propel it forward^[18][19]. Scott W. Simpson, a professor of anatomy at Case Western Reserve University School of Medicine, led research on *Ardipithecus ramidus* fossils^[18][19]. He stated, "Our research shows that while *Ardipithecus* was a lousy biped, she was somewhat better than we thought before"^[18][19]. This suggests a gradual transition to fully modern human walking^[18][20]. The *Ardipithecus* foot was more rigid than chimpanzees, which improved propulsion on the ground^[17]. Later, *Australopithecus afarensis*, known through the "Lucy" skeleton, lived between 3.7 and 3 million years ago^[21][22]. Lucy's skeletal features, particularly her pelvis and knee joints, show clear adaptations for habitual bipedalism^[21][23]. Her foot bones indicate a permanently arched foot, supporting a modern-human style of locomotion by 3.2 million years ago^[21]. This species demonstrated habitual upright walking long before larger brains evolved^[22]. ### Why Our Ancestors Stood Upright The evolution of bipedalism was a complex process^[15][24]. Scientists propose several theories for why human ancestors began walking on two legs^[25][24]. These theories are not mutually exclusive, and multiple factors likely contributed to this major shift^[24]. One long-standing idea is the "savanna hypothesis"^[25][24]. This theory suggests that as forests shrank and open grasslands expanded, hominins needed to see over tall grasses to spot predators or food^[25][26]. Bipedalism would have provided an elevated vantage point^[27][28]. However, recent findings, including the wooded environments where early bipeds like *Ardipithecus* lived, challenge this idea^[16][29]. Some research suggests bipedalism may have evolved in the trees first^[24][29]. Another significant advantage is energy efficiency^[27][28]. Walking on two legs is more energetically efficient than quadrupedalism, especially for covering long distances^[27][28]. This would have been beneficial for traveling between scattered food sources^[28][26]. Studies on chimpanzees show they use 75 percent more energy walking bipedally than humans^[30]. Bipedalism also freed the hands^[27][28]. This allowed early hominins to carry food, infants, and eventually, tools and weapons^[27][28][31]. This ability to carry objects likely played a crucial role in developing complex behaviors and social structures^[28][31]. Charles Darwin first suggested this in 1871, though stone tools appeared much later than the origin of bipedalism^[30][32]. Thermoregulation is another proposed benefit^[26][32]. Standing upright exposes less of the body to direct sunlight, helping to reduce overheating in hot climates^[26][32]. This could have reduced water loss and allowed ancestors to forage longer^[32]. The shift to bipedalism involved significant anatomical changes over millions of years^[28][33]. The pelvis became shorter and broader to support the upper body^[28][33]. The human spine developed an S-shaped curvature for balance and shock absorption^[28]. The femur angled inward for better weight distribution, and feet evolved with a larger heel and developed arches for shock absorption and propulsion^[28][34]. These changes allowed the body to balance on one leg while the other took a step^[34]. The ability to walk upright marks a pivotal moment in human evolution^[4][28]. It allowed our ancestors to adapt to changing environments and paved the way for other uniquely human traits^[28]. While the exact drivers are still debated, the evidence shows this fundamental change began much earlier than previously confirmed^[2].

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