Podcast Summary: Más de uno – ¿Cuándo empezamos a caminar a dos patas? Una nueva pieza del puzle, el desconocido Sahelanthropus

Host: Carlos Alsina (A) | Guest Expert: Alberto Aparici (B)
Date: January 15, 2026 | Podcast: OndaCero

Main Theme

This episode dives into the evolutionary mystery of bipedalism—when and why humans began to walk upright on two legs. Carlos Alsina and Alberto Aparici explore the most recent scientific findings, focusing on the ancient hominid Sahelanthropus, and reveal how new fossil analysis is reshaping our understanding of human evolution.

Key Discussion Points & Insights

1. Setting the Stage: The Upright Puzzle

• [01:03] Alberto introduces a mysterious sign found on his library door: "Recuerden entrar bien erguidos, no se olviden de dónde vienen," setting up the theme of human uprightness.
• The group embarks on their typical “library” journey, discovering books on human evolution.

2. The Evolutionary Timeline of Bipedalism

• [03:07]–[03:51] quote reading from Jeremy de Silva’s "Paso a paso" about chimpanzee vs. human walking, highlighting our unique efficient but slower two-legged gait:

  "Mantienen las rodillas flexionadas y la espalda encorvada... Caminamos con mucho más garbo que un chimpancé. Y además gastamos menos energía. Somos eficientes. Eficientes sí, pero rápidos, no. El mismísimo Usain Bolt no podría escapar de un león ni atrapar un conejo. Al apostar por las dos patas, renunciamos a algo que otros animales sí: el galope." (D, 03:10)

• A recent scientific article brings a crucial update: we are closer to knowing when and why we stood up ("Una pieza más de este puzle...").

3. Shifting Theories: From Savannas to Ancient Trees

• Classic hypothesis: Our ancestors became bipedal after leaving the trees due to African climate change (~8 million years ago).
  • Bipedalism was thought to help with seeing over tall grass, carrying infants, or chasing prey, but these were later questioned (“historietas”).
  • Now, the question becomes: Did we become bipeds because we left trees, or could we leave trees because we were already bipeds? ("¿Se hicieron bípedos nuestros antepasados porque bajaron de los árboles o... podían bajar de los árboles porque antes ya caminaban sobre dos patas?”) [05:17]

4. The Evidence: Famous Fossils & Their Lessons

• Lucy (Australopithecus, discovered 1974):

  • [06:10] Lucy’s knee and leg bones show a definitive bipedal adaptation (“ángulo valgo de la rodilla”).
  • Until Lucy, scientists believed bipedalism was far more recent.

• Ardipithecus (found in 1990s):

  • Dated to 4.4 million years ago.
  • Had a grasping big toe (arboreal adaptation) and evidence of bipedalism.
  • Shows early bipedalism doesn't necessarily mean terrestrial life.

• Pelvis vs. Leg Bones:

  • Legs provide clearer bipedal evidence than pelvis alone, though both are related. ("Lo ideal es mirar las piernas. Ahí es donde está la información.”) [09:30]

5. The Enigmatic Sahelanthropus: A New Clue

• Fossil Gap: A “black hole” in the fossil record between 5–14 million years ago.

  • Exception: Sahelanthropus discovered in Chad, 2001—ca. 7 million years old. [10:00]
  • Only a cranium was found at first (“cráneo del mismo tamaño que el de un chimpancé”), but the position of the foramen magno (spinal hole) suggests upright posture.

  "Nosotros como caminamos erguidos, tenemos el foramen magno debajo del cráneo... un perro lo tiene en la parte de atrás." (B, 10:57)

• Recent Breakthrough:

  • A fragmentary femur (“el trozo de un fémur”) found in 2001 but only analyzed recently.
  • Subtle evidence: A pronounced attachment for the iliofemoral ligament, crucial for upright walking (“el ligamento más largo y más fuerte del cuerpo humano”) [12:00]
  • Suggests adaptation for bipedalism at least 7 million years ago—even as the African forests were vanishing.

• Remaining Questions:

  • No foot bones: Unsure if Sahelanthropus had a grasping toe, so whether it was mainly arboreal is unclear.
  • No complete knee: Hard to compare to Australopithecus bipedal features.

6. A Historic “Culebrón” (Scientific Drama)

• [14:48] The "culebrón" of the Sahelanthropus femur: Found in 2001, it was mistaken for an animal bone.
  • In 2004, a student recognized its importance, but the excavation leader, Michel Brunet, withheld access for years, delaying publication until 2020 and thorough study until 2026.

  “El líder del grupo es una persona muy posesiva, muy problemática. [...] Y a partir de ese momento decidió que no le iba a enseñar el fémur a nadie...” (B, 15:13)

Notable Quotes & Moments

• On science communication (meta):

  “Habló muchísimo de lo mal que hablamos en los medios de comunicación de la ciencia.” (A, 00:23)

• On efficiency vs. speed in evolution:

  “El mismísimo Usain Bolt no podría escapar de un león ni atrapar un conejo... renunciamos a lo que otros animales sí: el galope.” (D, 03:17)

• On discovering our bipedal roots:

  “El registro fósil nos hace un regate y hay una especie de agujero negro entre hace 5 millones de años y hace 14 millones de años.” (B, 10:00)

• On scientific drama:

  “Un científico francés que aún vive, tiene ochenta y pico años... decidió que no le iba a enseñar el fémur a nadie...” (B, 15:10)

Timeline of Important Segments

| Timestamp | Segment Summary | |-----------|----------------| | 01:03 | Introduction to the show's core theme: "erguidos" humans | | 03:07–03:51 | Reading from "Paso a paso" on human vs chimpanzee walking habits | | 05:17 | Questioning the link between loss of forests and bipedalism | | 06:10 | Analysis of Lucy (Australopithecus) | | 08:12 | Introduction of Ardipithecus | | 10:00 | The fossil gap and Sahelanthropus introduction | | 10:57 | Foramen magno as evidence of bipedalism | | 12:00 | The recent femur study and ligament evidence | | 14:48 | The scientific “culebrón” about the Sahelanthropus femur |

Tone & Style

The episode combines scientific rigor, clear explanations, and lighthearted banter—Alberto Aparici demystifies complex paleontological topics in a spirit of curiosity and gentle humor, while the hosts interject with practical questions, asides, and witty remarks ("Enviamos un saludo cariñoso desde aquí a este francés...").

Conclusion

The episode traces humanity’s long, tangled road to walking upright, spotlighting the pivotal yet fragmentary evidence locked in ancient bones. With the latest analysis on Sahelanthropus, science continues to piece together the origins of bipedalism, but the puzzle is far from complete—calling for more fossils, fewer scientific rivalries, and ongoing curiosity.